lint: Allow revert commit messages in gitlint.

Fixes #8145.

.gitlint

@@ -1,9 +1,9 @@
 [general]
-ignore=title-trailing-punctuation, body-min-length, body-is-missing
+ignore=title-trailing-punctuation, body-min-length, body-is-missing, title-imperative-mood
 
-# extra-path=tools/lib/gitlint-rules.py
+extra-path=tools/lib/gitlint-rules.py
 
-[title-match-regex]
+[title-match-regex-allow-exception]
 regex=^(.+:\ )?[A-Z].+\.$
 
 [title-max-length]

tools/lib/gitlint-rules.py

@@ -2,6 +2,8 @@ from typing import Text, List
 
 import gitlint
 from gitlint.rules import LineRule, RuleViolation, CommitMessageTitle
+from gitlint.options import StrOption
+import re
 
 # Word list from https://github.com/m1foley/fit-commit
 # Copyright (c) 2015 Mike Foley
@@ -141,3 +143,23 @@ class ImperativeMood(LineRule):
             violations.append(violation)
 
         return violations
+
+
+class TitleMatchRegexAllowException(LineRule):
+    """Allows revert commits contrary to the built-in title-match-regex rule"""
+
+    name = 'title-match-regex-allow-exception'
+    id = 'Z2'
+    target = CommitMessageTitle
+    options_spec = [StrOption('regex', ".*", "Regex the title should match")]
+
+    def validate(self, title, commit):
+        # type: (Text, gitlint.commit) -> List[RuleViolation]
+
+        regex = self.options['regex'].value
+        pattern = re.compile(regex, re.UNICODE)
+        if not pattern.search(title) and not title.startswith("Revert \""):
+            violation_msg = u"Title does not match regex ({0})".format(regex)
+            return [RuleViolation(self.id, violation_msg, title)]
+
+        return []