checkpatch: fix patch separator line regex

The separator line always starts with three dashes on a line, optionally
followed by either white-space, OR a single space and a filename.  The
regex would previously match on any three dashes in a row.  This means
that a patch (such as [1]) would trigger the parser state machine to
advance beyond the signed-off checks.

Now, bound the check only to use what git-mailinfo would use as a
separator.
   --- <filename>
   ---<sp>

1: https://mail.openvswitch.org/pipermail/ovs-dev/2018-June/348625.html

Fixes: c599d5ccf3 ("checkpatch.py: A simple script for finding patch issues")
Signed-off-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

utilities/checkpatch.py

@@ -619,7 +619,7 @@ def ovs_checkpatch_parse(text, filename):
     parse = 0
     current_file = filename if checking_file else ''
     previous_file = ''
-    scissors = re.compile(r'^[\w]*---[\w]*')
+    seppatch = re.compile(r'^---([\w]*| \S+)$')
     hunks = re.compile('^(---|\+\+\+) (\S+)')
     hunk_differences = re.compile(
         r'^@@ ([0-9-+]+),([0-9-+]+) ([0-9-+]+),([0-9-+]+) @@')
@@ -652,7 +652,7 @@ def ovs_checkpatch_parse(text, filename):
                 print_file_name = current_file
             continue
         elif parse == PARSE_STATE_HEADING:
-            if scissors.match(line):
+            if seppatch.match(line):
                 parse = PARSE_STATE_DIFF_HEADER
                 if not skip_signoff_check:
                     if len(signatures) == 0: