Implement the equivalent of git/mailinfo.c:cleanup_subject() (i.e., remove [PATCH] from title)

[gwd]

Patches generated with git format-patch will have [PATCH] in the subject line; at the moment, this doesn't seem to be removed by ParsePatchHeader().

It might be a good idea to just implement the equivalent of https://github.com/git/git/blob/master/mailinfo.c 's cleanup_subject(), which seems to remove the following things at the beginning of the patch title:

1. Re: and variations
2. Whitespace (' ', '\t', and ':')
3. Anything in between brackets.

I might take at implementing this if I have time.