Handle unicode emails in git commits

This should probably never happen, but it has anyway, so assume the
email field is a utf8 string.  This should avoid crashing in case
someone has a utf8 email, but may not be a complete solution to
the case that an email is in a non-ascii, non-utf8 charset.

Change-Id: I95ed91e5fdfe35b73550e1824e609a0b1d388b3a