check-commit: use raw string for regular expression

Avoids SyntaxWarning on Python 3.8.

Differential Revision: https://phab.mercurial-scm.org/D5824

contrib/check-commit

@@ -47,7 +47,7 errors = [
      "adds a function with foo_bar naming"),
 ]
 
-word = re.compile('\S')
+word = re.compile(r'\S')
 def nonempty(first, second):
     if word.search(first):
         return first