checkstyle: Add support for checking style on amendments

This introduces a new argument "--amend" and a new special type of
commit "Amendment". It will check the style of changes that are in
the index combined with the changes of the last commit. So this is
the changes that would be applied by "git commit --amend" hence the
name of the argument.

This is needed to implement pre-commit hook.

Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

utils/checkstyle.py

@@ -496,6 +496,26 @@ class StagedChanges(Commit):
                               stdout=subprocess.PIPE).stdout.decode('utf-8')
 
 
+class Amendment(StagedChanges):
+    def __init__(self):
+        StagedChanges.__init__(self)
+
+    def get_info(self):
+        # Create a title using HEAD commit
+        ret = subprocess.run(['git', 'show', '--pretty=oneline', '--no-patch'],
+                             stdout=subprocess.PIPE).stdout.decode('utf-8')
+        title = 'Amendment of ' + ret.strip()
+        # Extract the list of modified files
+        ret = subprocess.run(['git', 'diff', '--staged', '--name-only', 'HEAD~'],
+                             stdout=subprocess.PIPE).stdout.decode('utf-8')
+        return title, ret.splitlines()
+
+    def get_diff(self, top_level, filename):
+        return subprocess.run(['git', 'diff', '--staged', 'HEAD~', '--',
+                               '%s/%s' % (top_level, filename)],
+                              stdout=subprocess.PIPE).stdout.decode('utf-8')
+
+
 def check_file(top_level, commit, filename):
     # Extract the line numbers touched by the commit.
     diff = commit.get_diff(top_level, filename)
@@ -628,6 +648,8 @@ def main(argv):
                         help='Code formatter. Default to clang-format if not specified.')
     parser.add_argument('--staged', '-s', action='store_true',
                         help='Include the changes in the index. Defaults to False')
+    parser.add_argument('--amend', '-a', action='store_true',
+                        help='Include changes in the index and the previous patch combined. Defaults to False')
     parser.add_argument('revision_range', type=str, default=None, nargs='?',
                         help='Revision range (as defined by git rev-parse). Defaults to HEAD if not specified.')
     args = parser.parse_args(argv[1:])
@@ -666,8 +688,10 @@ def main(argv):
     commits = []
     if args.staged:
         commits.append(StagedChanges())
+    if args.amend:
+        commits.append(Amendment())
 
-    # If not --staged
+    # If none of --staged or --amend was passed
     if len(commits) == 0:
         # And no revisions were passed, then default to HEAD
         if not args.revision_range: