[clang-format] Add "three dot" diff option to git-clang-format

This patch adds in the ability to do a "three dot" git-clang-format
between two commits. This looks at the diff between the second commit
and the common merge base rather than comparing at the point of the
specified commits. This is needed to improve the reliability of the LLVM
code formatting CI action which currently breaks in some cases where
files have been modified in the upstream tree and when the person
created their branch, leaving phantom formatting diffs that weren't
touched by the PR author.

Part of a fix for llvm#73873

Author: boomanaiden154

clang/tools/clang-format/git-clang-format

@@ -132,6 +132,10 @@ def main():
                  help='passed to clang-format'),
   p.add_argument('-v', '--verbose', action='count', default=0,
                  help='print extra information')
+  p.add_argument('--diff_from_common_commit', action='store_true',
+                 help=('diff from the last common commit for commits in '
+                      'separate branches rather than the exact point of the '
+                      'commits'))
   # We gather all the remaining positional arguments into 'args' since we need
   # to use some heuristics to determine whether or not <commit> was present.
   # However, to print pretty messages, we make use of metavar and help.
@@ -154,7 +158,10 @@ def main():
     if len(commits) > 2:
       die('at most two commits allowed; %d given' % len(commits))
   opts.binary=os.path.abspath(opts.binary)
-  changed_lines = compute_diff_and_extract_lines(commits, files, opts.staged)
+  changed_lines = compute_diff_and_extract_lines(commits,
+                                                 files,
+                                                 opts.staged,
+                                                 opts.diff_from_common_commit)
   if opts.verbose >= 1:
     ignored_files = set(changed_lines)
   filter_by_extension(changed_lines, opts.extensions.lower().split(','))
@@ -302,9 +309,9 @@ def get_object_type(value):
   return convert_string(stdout.strip())
 
 
-def compute_diff_and_extract_lines(commits, files, staged):
+def compute_diff_and_extract_lines(commits, files, staged, diff_common_commit):
   """Calls compute_diff() followed by extract_lines()."""
-  diff_process = compute_diff(commits, files, staged)
+  diff_process = compute_diff(commits, files, staged, diff_common_commit)
   changed_lines = extract_lines(diff_process.stdout)
   diff_process.stdout.close()
   diff_process.wait()
@@ -314,7 +321,7 @@ def compute_diff_and_extract_lines(commits, files, staged):
   return changed_lines
 
 
-def compute_diff(commits, files, staged):
+def compute_diff(commits, files, staged, diff_common_commit):
   """Return a subprocess object producing the diff from `commits`.
 
   The return value's `stdin` file object will produce a patch with the
@@ -328,6 +335,10 @@ def compute_diff(commits, files, staged):
     git_tool = 'diff-tree'
   elif staged:
     extra_args += ['--cached']
+
+  if len(commits) > 1 and diff_common_commit:
+    commits = [f'{commits[0]}...{commits[1]}']
+
   cmd = ['git', git_tool, '-p', '-U0'] + extra_args + commits + ['--']
   cmd.extend(files)
   p = subprocess.Popen(cmd, stdin=subprocess.PIPE, stdout=subprocess.PIPE)