user map to support equal sign

[Elan Ruusamäe]

in my bzr repo, i have some commits where author is QuotedPrintable encoded, and with current implementation it's not possible to remap these

user_mapper.py def _parse(self, lines):

            old, new = line.split('=', 1)

how about splitting with equal sign from the end:

            old = "=".join(line.split('=')[:-1]).strip()
            new = "=".join(line.split('=')[-1:]).strip()

then i could put to map file:
=?utf-8?q?Elan_Ruusam=C3=A4e?= <email address hidden>=Elan Ruusamäe <email address hidden>

[Elan Ruusamäe (glen666)]

proposed patch