rebase: fix broken email with --committer-date-is-author-date

rebase: fix broken email with --committer-date-is-author-date

Commit 7573cec52c (rebase -i: support --committer-date-is-author-date,
2020-08-17) copied the committer ident-parsing code from builtin/am.c.
And in doing so, it copied a bug in which we always set the email to an
empty string. We fixed the version in git-am in the previous commit;
this commit fixes the copied code.

Reported-by: VenomVendor <redacted>
Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>