rebase -i: improve error message when picking merge

The only todo commands that accept a merge commit are "merge" and
"reset". All the other commands like "pick" or "reword" fail when they
try to pick a a merge commit and print the message

    error: commit abc123 is a merge but no -m option was given.

followed by a hint about the command being rescheduled. This message is
designed to help the user when they cherry-pick a merge and forget to
pass "-m". For users who are rebasing the message is confusing as there
is no way for rebase to cherry-pick the merge.

Improve the user experience by detecting the error and printing some
advice on how to fix it when the todo list is parsed rather than waiting
for the "pick" command to fail. The advice recommends "merge" rather
than "exec git cherry-pick -m ..." on the assumption that cherry-picking
merges is relatively rare and it is more likely that the user chose
"pick" by a mistake.

It would be possible to support cherry-picking merges by allowing the
user to pass "-m" to "pick" commands but that adds complexity to do
something that can already be achieved with

    exec git cherry-pick -m1 abc123

Reported-by: Stefan Haller <lists@haller-berlin.de>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 45 insertions, 0 deletions

diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
index d1bead61fa..f92baad138 100755
--- a/t/t3404-rebase-interactive.sh
+++ b/t/t3404-rebase-interactive.sh
@@ -2215,6 +2215,51 @@ test_expect_success 'bad labels and refs rejected when parsing todo list' '
 	test_path_is_missing execed
 '
 
+test_expect_success 'non-merge commands reject merge commits' '
+	test_when_finished "test_might_fail git rebase --abort" &&
+	git checkout E &&
+	git merge I &&
+	oid=$(git rev-parse HEAD) &&
+	cat >todo <<-EOF &&
+	pick $oid
+	reword $oid
+	edit $oid
+	fixup $oid
+	squash $oid
+	EOF
+	(
+		set_replace_editor todo &&
+		test_must_fail git rebase -i HEAD 2>actual
+	) &&
+	cat >expect <<-EOF &&
+	error: ${SQ}pick${SQ} does not accept merge commits
+	hint: ${SQ}pick${SQ} does not take a merge commit. If you wanted to
+	hint: replay the merge, use ${SQ}merge -C${SQ} on the commit.
+	hint: Disable this message with "git config advice.rebaseTodoError false"
+	error: invalid line 1: pick $oid
+	error: ${SQ}reword${SQ} does not accept merge commits
+	hint: ${SQ}reword${SQ} does not take a merge commit. If you wanted to
+	hint: replay the merge and reword the commit message, use
+	hint: ${SQ}merge -c${SQ} on the commit
+	hint: Disable this message with "git config advice.rebaseTodoError false"
+	error: invalid line 2: reword $oid
+	error: ${SQ}edit${SQ} does not accept merge commits
+	hint: ${SQ}edit${SQ} does not take a merge commit. If you wanted to
+	hint: replay the merge, use ${SQ}merge -C${SQ} on the commit, and then
+	hint: ${SQ}break${SQ} to give the control back to you so that you can
+	hint: do ${SQ}git commit --amend && git rebase --continue${SQ}.
+	hint: Disable this message with "git config advice.rebaseTodoError false"
+	error: invalid line 3: edit $oid
+	error: cannot squash merge commit into another commit
+	error: invalid line 4: fixup $oid
+	error: cannot squash merge commit into another commit
+	error: invalid line 5: squash $oid
+	You can fix this with ${SQ}git rebase --edit-todo${SQ} and then run ${SQ}git rebase --continue${SQ}.
+	Or you can abort the rebase with ${SQ}git rebase --abort${SQ}.
+	EOF
+	test_cmp expect actual
+'
+
 # This must be the last test in this file
 test_expect_success '$EDITOR and friends are unchanged' '
 	test_editor_unchanged