test/contrib: Fix invalid escapes in regex strings

Flagged by flake8 v3.6.0, as W605, plus a few others identified
incidentally, e.g. 59ffecf66cf4d08c4b431e457b083878d66a3fd6.

Note that r"\n" matches to "\n" under re.match/search.

contrib/devtools/clang-format-diff.py

@@ -106,7 +106,7 @@ def main():
   filename = None
   lines_by_file = {}
   for line in sys.stdin:
-    match = re.search('^\+\+\+\ (.*?/){%s}(\S*)' % args.p, line)
+    match = re.search(r'^\+\+\+\ (.*?/){%s}(\S*)' % args.p, line)
     if match:
       filename = match.group(2)
     if filename is None:
@@ -119,7 +119,7 @@ def main():
       if not re.match('^%s$' % args.iregex, filename, re.IGNORECASE):
         continue
 
-    match = re.search('^@@.*\+(\d+)(,(\d+))?', line)
+    match = re.search(r'^@@.*\+(\d+)(,(\d+))?', line)
     if match:
       start_line = int(match.group(1))
       line_count = 1