crecord: use raw string for regular expression

\s emits a SyntaxWarning in Python 3.8. Use a raw string to
avoid escaping the \.

Differential Revision: https://phab.mercurial-scm.org/D5819

mercurial/crecord.py

@@ -1799,6 +1799,7 b' are you sure you want to review/edit and'
                 break
 
         if self.commenttext != "":
-            whitespaceremoved = re.sub("(?m)^\s.*(\n|$)", "", self.commenttext)
+            whitespaceremoved = re.sub(br"(?m)^\s.*(\n|$)", b"",
+                                       self.commenttext)
             if whitespaceremoved != "":
                 self.opts['message'] = self.commenttext