am, sequencer: stop parsing our own committer ident

am, sequencer: stop parsing our own committer ident

For the --committer-date-is-author-date option of git-am and git-rebase,
we format the committer ident, then re-parse it to find the name and
email, and then feed those back to fmt_ident().

We can simplify this by handling it all at the time of the fmt_ident()
call. We pass in the appropriate getenv() results, and if they're not
present, then our WANT_COMMITTER_IDENT flag tells fmt_ident() to fill in
the appropriate value from the config. Which is exactly what
git_committer_ident() was doing under the hood.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>