Fix FutureWarning about nested sets in re

Since Python 3.7 the re module warns for syntax which could, in the
future, be misparsed as a nested set. Avoid this by escaping the
literal `[` we search for in the regexp.

Reported by Monte Davidoff @mndavidoff

Closes #269.

hg-fast-export.py

@@ -266,7 +266,7 @@ def sanitize_name(name,what="branch", mapping={}):
   if not auto_sanitize:
     return mapping.get(name,name)
   n=mapping.get(name,name)
-  p=re.compile(b'([[ ~^:?\\\\*]|\.\.)')
+  p=re.compile(b'([\\[ ~^:?\\\\*]|\.\.)')
   n=p.sub(b'_', n)
   if n[-1:] in (b'/', b'.'): n=n[:-1]+b'_'
   n=b'/'.join([dot(s) for s in n.split(b'/')])