Commit e8cbe211 (am: stop exporting GIT_COMMITTER_DATE, 2020-08-17)
rewrote the code for setting the committer date to use fmt_ident(),
rather than setting an environment variable and letting commit_tree()
handle it. But it introduced two bugs:

  - we use the author email string instead of the committer email

  - when parsing the committer ident, we used the wrong variable to
    compute the length of the email, resulting in it always being a
    zero-length string

This commit fixes both, which causes our test of this option via the
rebase "apply" backend to now succeed.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>