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

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>

builtin/am.c

@@ -161,7 +161,7 @@ static void am_state_init(struct am_state *state)
 	state->committer_name =
 		xmemdupz(id.name_begin, id.name_end - id.name_begin);
 	state->committer_email =
-		xmemdupz(id.mail_begin, id.mail_end - id.mail_end);
+		xmemdupz(id.mail_begin, id.mail_end - id.mail_begin);
 }
 
 /**
@@ -1595,7 +1595,7 @@ static void do_commit(const struct am_state *state)
 
 	if (state->committer_date_is_author_date)
 		committer = fmt_ident(state->committer_name,
-				      state->author_email, WANT_COMMITTER_IDENT,
+				      state->committer_email, WANT_COMMITTER_IDENT,
 				      state->ignore_date ? NULL
 							 : state->author_date,
 				      IDENT_STRICT);

t/t3436-rebase-more-options.sh

@@ -70,7 +70,7 @@ test_ctime_is_atime () {
 	test_cmp authortime committertime
 }
 
-test_expect_failure '--committer-date-is-author-date works with apply backend' '
+test_expect_success '--committer-date-is-author-date works with apply backend' '
 	GIT_AUTHOR_DATE="@1234 +0300" git commit --amend --reset-author &&
 	git rebase --apply --committer-date-is-author-date HEAD^ &&
 	test_ctime_is_atime -1