utils: checkstyle.py: Support single line hunks

The checkstyle script expects hunks to be declared with a start line and
line count, however the unified diff format [0] declares that a single
line hunk will only have the start line:

> If a hunk contains just one line, only its start line number appears.
> Otherwise its line numbers look like ‘start,count’. An empty hunk is
> considered to start at the line that follows the hunk.

[0] https://www.gnu.org/software/diffutils/manual/html_node/Detailed-Unified.html#Detailed-Unified

Attempting to parse a single line hunk results in the following error:

  File "./utils/checkstyle.py", line 110, in __init__
    raise RuntimeError("Malformed diff hunk header '%s'" % line)
  RuntimeError: Malformed diff hunk header '@@ -1 +1,2 @@

The DiffHunk class only makes use of the start line, and does not
utilise the line count, thus update the regex to make the unused
groups optional.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

utils/checkstyle.py

@@ -102,7 +102,7 @@ class DiffHunkSide(object):
 
 
 class DiffHunk(object):
-    diff_header_regex = re.compile(r'@@ -([0-9]+),([0-9]+) \+([0-9]+),([0-9]+) @@')
+    diff_header_regex = re.compile(r'@@ -([0-9]+),?([0-9]+)? \+([0-9]+),?([0-9]+)? @@')
 
     def __init__(self, line):
         match = DiffHunk.diff_header_regex.match(line)