Use a System clang-tidy When Available and Make --fix Converge in One Pass (#16199)

* Use a System clang-tidy When Available and Make --fix Converge in One Pass

scripts/run_clang_tidy.sh only looked at CLANG_TIDY and the venv it creates,
so a clang-tidy already on the system was never used. It is now the first
choice: the pinned version outright, another version after a prompt that
says results may differ slightly from CI, which -y and an existing pinned
venv skip.

Two problems in clang_tidy_diff.py made --fix need several runs and still
leave the plain check failing:

- A deleted #include orphans uses on unchanged lines. The plain check runs
  such a file whole and reports them, but --fix kept the line filter to the
  changed lines, so they were never fixed. Fix mode now runs the file whole
  first and then fixes exactly the changed lines plus the lines that run
  found wanting, so unrelated lines are still never rewritten.

- clang-tidy exits non-zero for the findings it just fixed, so every fixed
  file was reported as failed and the user ran --fix again to see what was
  left. A file --fix changed is now checked again and the fixed files are
  listed separately from what --fix could not add.

CI runs the script without --fix and is unchanged.

* Keep the a/ b/ Diff Prefixes Whatever the User's Git Config Says

parse_diff recognises a changed file by its +++ b/ header. With
diff.noprefix or diff.mnemonicPrefix set, git prints +++ src/x.cpp or
+++ w/src/x.cpp instead, every file was dropped, and the local check
reported no changed C++ lines. The diff is now asked for the a/ and b/
prefixes outright, which overrides both settings.

* Warn When No Remote Points at OrcaSlicer/OrcaSlicer

Without one, run_clang_tidy.sh compares against origin/main. When origin
is a fork whose main already holds the commits, the check finds nothing
and says so, without hinting at why. The script now names the base it
fell back to and how to point it at the upstream repository.

scripts/clang_tidy_diff.py

@@ -8,6 +8,9 @@ include) fails wherever the error is. Headers are compiled on their own, and
 fail only on errors in their changed lines. Deleting an #include also fails
 on every use, changed or not, that now lacks the header it provided.
 
+With -- --fix, clang-tidy adds the missing includes, on those lines only, and
+each file it changed is checked again so that only what remains is reported.
+
 The checks come from .clang-tidy at the repository root. The compile database
 must come from a configure with SLIC3R_PCH=OFF, or the precompiled header hides
 missing includes.
@@ -102,9 +105,10 @@ def is_checked(path):
 
 def changed_files(merge_base):
     # Against the working tree, so a local run covers uncommitted edits too.
-    # core.quotePath=false keeps a non-ASCII path unquoted, so parse_diff sees its b/ prefix.
+    # core.quotePath=false keeps a non-ASCII path unquoted, and the explicit prefixes
+    # override diff.noprefix and diff.mnemonicPrefix, so parse_diff sees its b/ prefix.
     diff = subprocess.run(["git", "-c", "core.quotePath=false", "diff", "-U0", "--no-color", "--no-ext-diff",
-                           "--diff-filter=AMR", merge_base],
+                           "--src-prefix=a/", "--dst-prefix=b/", "--diff-filter=AMR", merge_base],
                           check=True, capture_output=True, **UTF8).stdout
     return {path: change for path, change in parse_diff(diff).items() if is_checked(path)}
 
@@ -201,13 +205,19 @@ def errors_alone_at(revision, clang_tidy, build_dir, path):
 
 
 def check_file(clang_tidy, build_dir, merge_base, path, change, extra_args):
-    """Run clang-tidy on one file and return (failed, output, failing diagnostics)."""
+    """Run clang-tidy on one file and return (failed, output, failing diagnostics, fixed)."""
+    fixing = any(arg.startswith("--fix") for arg in extra_args)
+    if fixing:
+        with open(path, "rb") as f:
+            before = f.read()
     # A deleted include can orphan uses on unchanged lines, so such a file is
-    # checked whole and the findings narrowed here. --fix keeps the line filter
-    # so it never rewrites unrelated code.
-    whole = bool(change.removed_includes) and not extra_args
+    # checked whole and the findings narrowed here. --fix keeps a line filter so
+    # it never rewrites unrelated code, which for such a file means a second,
+    # fixing run limited to the lines the first one found wanting.
+    whole = bool(change.removed_includes)
     returncode, output, diagnostics = run_clang_tidy(clang_tidy, build_dir, path,
-                                                     None if whole else change.lines, extra_args)
+                                                     None if whole else change.lines,
+                                                     [] if whole else extra_args)
     real = os.path.realpath(path)
 
     def introduced(d):
@@ -223,14 +233,25 @@ def check_file(clang_tidy, build_dir, merge_base, path, change, extra_args):
         if errors and change.removed_includes:
             with open(path, encoding="utf-8") as f:
                 text = f.read()
-            before = errors_alone_at(merge_base, clang_tidy, build_dir, path)
+            before_sites = errors_alone_at(merge_base, clang_tidy, build_dir, path)
             failing += [d for d in errors if d not in failing
-                        and (error_sites([d], text) - before)]
-        return bool(failing), output, failing
-    if whole:
+                        and (error_sites([d], text) - before_sites)]
+        failed = bool(failing)
+    elif whole:
         failing = [d for d in diagnostics if d.is_compile_error or introduced(d)]
-        return bool(failing), output, failing
-    return returncode != 0, output, diagnostics
+        failed = bool(failing)
+    else:
+        failing, failed = diagnostics, returncode != 0
+    if not fixing:
+        return failed, output, failing, False
+    if whole and failing:
+        lines = change.lines + [[d.line, d.line] for d in failing if os.path.realpath(d.file) == real]
+        run_clang_tidy(clang_tidy, build_dir, path, lines, extra_args)
+    with open(path, "rb") as f:
+        if f.read() == before:
+            return failed, output, failing, False
+    # Checked again, so what is reported is what the fixes left.
+    return check_file(clang_tidy, build_dir, merge_base, path, change, [])[:3] + (True,)
 
 
 def main():
@@ -263,11 +284,14 @@ def main():
     annotate = os.environ.get("GITHUB_ACTIONS") == "true"
     root = os.getcwd() + os.sep
     failed = []
+    fixed = []
     with ThreadPoolExecutor(max_workers=args.jobs) as pool:
         jobs = {path: pool.submit(check_file, args.clang_tidy, args.build_dir, merge_base, path, change, args.extra_args)
                 for path, change in todo}
         for path, job in jobs.items():
-            file_failed, output, diagnostics = job.result()
+            file_failed, output, diagnostics, file_fixed = job.result()
+            if file_fixed:
+                fixed.append(path)
             if not file_failed:
                 continue
             failed.append(path)
@@ -282,6 +306,13 @@ def main():
             if len(diagnostics) > MAX_REPORTED:
                 print(f"... and {len(diagnostics) - MAX_REPORTED} more")
 
+    if fixed:
+        print(f"\nAdded includes to {len(fixed)} file(s):")
+        for path in fixed:
+            print(f"    {path}")
+    if failed and fixed:
+        print(f"\nclang-tidy still fails on {len(failed)} file(s); the findings above are what --fix could not add.")
+        return 1
     if failed:
         print(f"\nclang-tidy failed on {len(failed)} file(s). Add the includes it names, or apply its "
               "suggestions locally with scripts/run_clang_tidy.sh --fix (scripts\\run_clang_tidy.ps1 -Fix on Windows).")