Index: tools/clang-format/git-clang-format
===================================================================
--- tools/clang-format/git-clang-format
+++ tools/clang-format/git-clang-format
@@ -35,9 +35,9 @@
 usage = 'git clang-format [OPTIONS] [<commit>] [--] [<file>...]'
 
 desc = '''
-Run clang-format on all lines that differ between the working directory
-and <commit>, which defaults to HEAD.  Changes are only applied to the working
-directory.
+Run clang-format on all lines that differ between the working directory and
+<commit> (which defaults to HEAD), or all lines that changed in a specific
+commit.  Changes are only applied to the working directory.
 
 The following git-config settings set the default of the corresponding option:
   clangFormat.binary
@@ -90,6 +90,9 @@
   p.add_argument('--commit',
                  default=config.get('clangformat.commit', 'HEAD'),
                  help='default commit to use if none is specified'),
+  p.add_argument('--single-commit', action='store_true',
+                 help=('run clang-format on a single commit instead of against '
+                       'the working tree')),
   p.add_argument('--diff', action='store_true',
                  help='print a diff instead of applying the changes')
   p.add_argument('--extensions',
@@ -121,7 +124,8 @@
   del opts.quiet
 
   commit, files = interpret_args(opts.args, dash_dash, opts.commit)
-  changed_lines = compute_diff_and_extract_lines(commit, files)
+  changed_lines = compute_diff_and_extract_lines(commit, files,
+                                                 opts.single_commit)
   if opts.verbose >= 1:
     ignored_files = set(changed_lines)
   filter_by_extension(changed_lines, opts.extensions.lower().split(','))
@@ -141,7 +145,10 @@
   # The computed diff outputs absolute paths, so we must cd before accessing
   # those files.
   cd_to_toplevel()
-  old_tree = create_tree_from_workdir(changed_lines)
+  if opts.single_commit:
+    old_tree = create_tree_from_commit(commit, changed_lines)
+  else:
+    old_tree = create_tree_from_workdir(changed_lines)
   new_tree = run_clang_format_and_save_to_tree(changed_lines,
                                                binary=opts.binary,
                                                style=opts.style)
@@ -242,9 +249,9 @@
   return stdout.strip()
 
 
-def compute_diff_and_extract_lines(commit, files):
+def compute_diff_and_extract_lines(commit, files, single_commit):
   """Calls compute_diff() followed by extract_lines()."""
-  diff_process = compute_diff(commit, files)
+  diff_process = compute_diff(commit, files, single_commit)
   changed_lines = extract_lines(diff_process.stdout)
   diff_process.stdout.close()
   diff_process.wait()
@@ -254,13 +261,16 @@
   return changed_lines
 
 
-def compute_diff(commit, files):
+def compute_diff(commit, files, single_commit):
   """Return a subprocess object producing the diff from `commit`.
 
   The return value's `stdin` file object will produce a patch with the
   differences between the working directory and `commit`, filtered on `files`
   (if non-empty).  Zero context lines are used in the patch."""
-  cmd = ['git', 'diff-index', '-p', '-U0', commit, '--']
+  git_tool = 'diff-index'
+  if single_commit:
+    git_tool = 'diff-tree'
+  cmd = ['git', git_tool, '-p', '-U0', commit, '--']
   cmd.extend(files)
   p = subprocess.Popen(cmd, stdin=subprocess.PIPE, stdout=subprocess.PIPE)
   p.stdin.close()
@@ -310,6 +320,32 @@
   os.chdir(toplevel)
 
 
+def create_tree_from_commit(commit, filenames):
+  """Create a new git tree with the given files from `commit`.
+
+  This reduces the size of the generated temporary index in case the project has
+  a large number of files.
+
+  Returns the object ID (SHA-1) of the created tree."""
+  def index_info_generator(lines, filenames):
+    for line in lines:
+      match = re.match(r'^(\d+) blob ([0-9a-f]+)\t(.*)', line)
+      if not match:
+        continue
+      mode, blob_id, filename = match.groups()
+      if filename not in filenames:
+        continue
+      yield '%s %s\t%s' % (mode, blob_id, filename)
+  cmd = ['git', 'ls-tree', '-r', commit]
+  p = subprocess.Popen(cmd, stdout=subprocess.PIPE)
+  stdout = p.communicate()[0]
+  if p.returncode != 0:
+    die('`%s` failed' % ' '.join(cmd))
+
+  return create_tree(index_info_generator(stdout.splitlines(), set(filenames)),
+                     '--index-info')
+
+
 def create_tree_from_workdir(filenames):
   """Create a new git tree with the given files from the working directory.