[Github] Fix revisions in code format action reproducers (#155193)

This patch makes it so the revisions that the code format action returns
in its reproducers actually work when applying them locally given the
differences in how revisions are setup in CI.

Fixes #154294

Author: boomanaiden154

llvm/utils/git/code-format-helper.py

@@ -92,6 +92,12 @@ def pr_comment_text_for_diff(self, diff: str) -> str:
 {self.instructions}
 ``````````
 
+:warning:
+The reproduction instructions above might return results for more than one PR
+in a stack if you are using a stacked PR workflow. You can limit the results by
+changing `origin/main` to the base branch/commit you want to compare against.
+:warning:
+
 </details>
 
 <details>
@@ -175,9 +181,32 @@ class ClangFormatHelper(FormatHelper):
     name = "clang-format"
     friendly_name = "C/C++ code formatter"
 
+    def _construct_command(self, diff_expression: list[str] | None):
+        cf_cmd = [self.clang_fmt_path, "--diff"]
+
+        if diff_expression:
+            cf_cmd.extend(diff_expression)
+
+        # Gather the extension of all modified files and pass them explicitly to git-clang-format.
+        # This prevents git-clang-format from applying its own filtering rules on top of ours.
+        extensions = set()
+        for file in self._cpp_files:
+            _, ext = os.path.splitext(file)
+            extensions.add(
+                ext.strip(".")
+            )  # Exclude periods since git-clang-format takes extensions without them
+        cf_cmd.append("--extensions")
+        cf_cmd.append(",".join(extensions))
+
+        cf_cmd.append("--")
+        cf_cmd += self._cpp_files
+        return cf_cmd
+
     @property
     def instructions(self) -> str:
-        return " ".join(self.cf_cmd)
+        # TODO(boomanaiden154): Add --diff_from_common_commit option when it has
+        # landed as in available in a released version.
+        return " ".join(self._construct_command(["origin/main", "HEAD"]))
 
     def should_include_extensionless_file(self, path: str) -> bool:
         return path.startswith("libcxx/include")
@@ -218,33 +247,19 @@ def has_tool(self) -> bool:
         return proc.returncode == 0
 
     def format_run(self, changed_files: List[str], args: FormatArgs) -> Optional[str]:
-        cpp_files = self.filter_changed_files(changed_files)
-        if not cpp_files:
+        self._cpp_files = self.filter_changed_files(changed_files)
+        if not self._cpp_files:
             return None
 
-        cf_cmd = [self.clang_fmt_path, "--diff"]
-
+        diff_expression = []
         if args.start_rev and args.end_rev:
-            cf_cmd.append(args.start_rev)
-            cf_cmd.append(args.end_rev)
-
-        # Gather the extension of all modified files and pass them explicitly to git-clang-format.
-        # This prevents git-clang-format from applying its own filtering rules on top of ours.
-        extensions = set()
-        for file in cpp_files:
-            _, ext = os.path.splitext(file)
-            extensions.add(
-                ext.strip(".")
-            )  # Exclude periods since git-clang-format takes extensions without them
-        cf_cmd.append("--extensions")
-        cf_cmd.append(",".join(extensions))
+            diff_expression.append(args.start_rev)
+            diff_expression.append(args.end_rev)
 
-        cf_cmd.append("--")
-        cf_cmd += cpp_files
+        cf_cmd = self._construct_command(diff_expression)
 
         if args.verbose:
             print(f"Running: {' '.join(cf_cmd)}")
-        self.cf_cmd = cf_cmd
         proc = subprocess.run(cf_cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
         sys.stdout.write(proc.stderr.decode("utf-8"))
 
@@ -263,9 +278,20 @@ class DarkerFormatHelper(FormatHelper):
     name = "darker"
     friendly_name = "Python code formatter"
 
+    def _construct_command(self, diff_expression: str | None) -> str:
+        darker_cmd = [
+            self.darker_fmt_path,
+            "--check",
+            "--diff",
+        ]
+        if diff_expression:
+            darker_cmd += ["-r", diff_expression]
+        darker_cmd += self._py_files
+        return darker_cmd
+
     @property
     def instructions(self) -> str:
-        return " ".join(self.darker_cmd)
+        return " ".join(self._construct_command("origin/main...HEAD"))
 
     def filter_changed_files(self, changed_files: List[str]) -> List[str]:
         filtered_files = []
@@ -295,17 +321,13 @@ def format_run(self, changed_files: List[str], args: FormatArgs) -> Optional[str
         py_files = self.filter_changed_files(changed_files)
         if not py_files:
             return None
-        darker_cmd = [
-            self.darker_fmt_path,
-            "--check",
-            "--diff",
-        ]
+        self._py_files = py_files
+        diff_expression = None
         if args.start_rev and args.end_rev:
-            darker_cmd += ["-r", f"{args.start_rev}...{args.end_rev}"]
-        darker_cmd += py_files
+            diff_expression = f"{args.start_rev}...{args.end_rev}"
+        darker_cmd = self._construct_command(diff_expression)
         if args.verbose:
             print(f"Running: {' '.join(darker_cmd)}")
-        self.darker_cmd = darker_cmd
         proc = subprocess.run(
             darker_cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE
         )