rebase -i: reword empty commit after fast-forward

When rebasing git tries to reuse existing commits by fast-forwarding if
the parents are unchanged. Rewording an empty commit that has been
fast-forwarded fails because "git commit --amend" is called without
"--allow-empty". This happens because when a commit is fast-forwarded
the logic that checks whether we should pass "--allow-empty" is
skipped. Fix this by always passing "--allow-empty" when rewording a
commit. This is safe when we fast-forward because we know that the
commit cannot have become empty due to the rebase as we are reusing an
existing commit object. It is also safe when we do not fast-forward
because the call to run_git_commit() that created the commit we're
rewording would have failed if the commit had become empty.

As "git commit" will happily create empty merge commits without
"--allow-empty" we do not need to pass that flag when rewording merge
commits.

Signed-off-by: Phillip Wood <[email protected]>

sequencer.c

@@ -2510,9 +2510,8 @@ static int do_pick_commit(struct repository *r,
 		*check_todo = !!(flags & EDIT_MSG);
 		if (!res && reword) {
 fast_forward_edit:
-			res = run_git_commit(NULL, opts, EDIT_MSG |
-					     VERIFY_MSG | AMEND_MSG |
-					     (flags & ALLOW_EMPTY));
+			flags = EDIT_MSG | VERIFY_MSG | AMEND_MSG | ALLOW_EMPTY;
+			res = run_git_commit(NULL, opts, flags);
 			*check_todo = 1;
 		}
 	}

t/t3404-rebase-interactive.sh

@@ -791,6 +791,20 @@ test_expect_success 'reword' '
 	grep "C changed" actual
 '
 
+test_expect_success 'reword fast-forwarded empty commit' '
+	git commit --allow-empty -m "empty commit" --only &&
+	(
+		set_fake_editor &&
+		FAKE_COMMIT_AMEND=edited FAKE_LINES="reword 1" \
+			git rebase -i HEAD^
+	) &&
+	test_commit_message HEAD <<-\EOF
+	empty commit
+
+	edited
+	EOF
+'
+
 test_expect_success 'no uncommitted changes when rewording and the todo list is reloaded' '
 	git checkout E &&
 	test_when_finished "git checkout @{-1}" &&

t/t3430-rebase-merges.sh

@@ -610,4 +610,24 @@ test_expect_success 'truncate label names' '
 	grep "label 0123456789-$" out
 '
 
+test_expect_success 'reword fast-forwarded empty merge commit' '
+	oid="$(git commit-tree -m "D1" -p A D^{tree})" &&
+	oid="$(git commit-tree -m "empty merge" -p D -p $oid D^{tree})" &&
+
+	write_script sequence-editor.sh <<-\EOF &&
+	sed /^merge/s/-C/-c/ "$1" >"$1.tmp"
+	mv "$1.tmp" "$1"
+	EOF
+
+	(
+		test_set_sequence_editor "$(pwd)/sequence-editor.sh" &&
+		GIT_EDITOR="echo edited >>" git rebase -i -r D $oid
+	) &&
+	test_commit_message HEAD <<-\EOF
+	empty merge
+
+	edited
+	EOF
+'
+
 test_done