Add option for specifying the text encoding used by Mercurial

When a mercurial repository does not use utf-8 for encoding author
strings and commit messages the "-e <encoding>" command line option
can be used to force fast-export to convert incoming meta data from
<encoding> to utf-8.

When "-e <encoding>" is given, we use Python's string
decoding/encoding API to convert meta data on the fly when processing
commits.

hg2git.py

@@ -67,9 +67,12 @@ def get_branch(name):
     return origin_name + '/' + name
   return name
 
-def get_changeset(ui,repo,revision,authors={}):
+def get_changeset(ui,repo,revision,authors={},encoding=''):
   node=repo.lookup(revision)
   (manifest,user,(time,timezone),files,desc,extra)=repo.changelog.read(node)
+  if encoding:
+    user=user.decode(encoding).encode('utf8')
+    desc=desc.decode(encoding).encode('utf8')
   tz="%+03d%02d" % (-timezone / 3600, ((-timezone % 3600) / 60))
   branch=get_branch(extra.get('branch','master'))
   return (node,manifest,fixup_user(user,authors),(time,tz),files,desc,branch,extra)