[clang-format-diff] Correctly parse start-of-file diffs

[tamird]

Summary

Handle the case where the diff is a pure removal of lines. Before this
change start_line would end up as 0 which is rejected by clang-format.

clang/tools/clang-format/clang-format-diff.py

for line in sys.stdin:
 
     if args.regex is not None:
       if not re.match('^%s$' % args.regex, filename):
         continue
     else:
       if not re.match('^%s$' % args.iregex, filename, re.IGNORECASE):
         continue
 
-    match = re.search(r'^@@.*\+(\d+)(,(\d+))?', line)
+    match = re.search(r'^@@.*\+(\d+)(?:,(\d+))?', line)
     if match:
       start_line = int(match.group(1))
       line_count = 1
-      if match.group(3):
-        line_count = int(match.group(3))
+      if match.group(2):
+        line_count = int(match.group(2))
+        # The input is something like
+        #
+        # @@ -1, +0,0 @@
+        #
+        # which means no lines were added.
+        if line_count == 0:
+          continue
       # Also format lines range if line_count is 0 in case of deleting
       # surrounding statements.
       end_line = start_line
       if line_count != 0:
         end_line += line_count - 1
       lines_by_file.setdefault(filename, []).extend(
           ['-lines', str(start_line) + ':' + str(end_line)])