rebase --continue: remove .git/MERGE_MSG

If the user skips the final commit by removing all the changes from
the index and worktree with 'git restore' (or read-tree) and then runs
'git rebase --continue' .git/MERGE_MSG is left behind. This will seed
the commit message the next time the user commits which is not what we
want to happen.

Reported-by: Victor Gambier <vgambier@excilys.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -4717,6 +4717,9 @@ static int commit_staged_changes(struct repository *r,
 		    refs_delete_ref(get_main_ref_store(r), "",
 				    "CHERRY_PICK_HEAD", NULL, 0))
 			return error(_("could not remove CHERRY_PICK_HEAD"));
+		if (unlink(git_path_merge_msg(r)) && errno != ENOENT)
+			return error_errno(_("could not remove '%s'"),
+					   git_path_merge_msg(r));
 		if (!final_fixup)
 			return 0;
 	}

t/t3403-rebase-skip.sh

@@ -20,6 +20,7 @@ test_expect_success setup '
 	git add hello &&
 	git commit -m "hello" &&
 	git branch skip-reference &&
+	git tag hello &&
 
 	echo world >> hello &&
 	git commit -a -m "hello world" &&
@@ -96,6 +97,13 @@ test_expect_success 'moved back to branch correctly' '
 
 test_debug 'gitk --all & sleep 1'
 
+test_expect_success 'skipping final pick removes .git/MERGE_MSG' '
+	test_must_fail git rebase --onto hello reverted-goodbye^ \
+		reverted-goodbye &&
+	git rebase --skip &&
+	test_path_is_missing .git/MERGE_MSG
+'
+
 test_expect_success 'correct advice upon picking empty commit' '
 	test_when_finished "git rebase --abort" &&
 	test_must_fail git rebase -i --onto goodbye \

t/t3418-rebase-continue.sh

@@ -31,6 +31,16 @@ test_expect_success 'merge based rebase --continue with works with touched file'
 	git rebase --continue
 '
 
+test_expect_success 'merge based rebase --continue removes .git/MERGE_MSG' '
+	git checkout -f --detach topic &&
+
+	test_must_fail git rebase --onto main HEAD^ &&
+	git read-tree --reset -u HEAD &&
+	test_path_is_file .git/MERGE_MSG &&
+	git rebase --continue &&
+	test_path_is_missing .git/MERGE_MSG
+'
+
 test_expect_success 'apply based rebase --continue works with touched file' '
 	rm -fr .git/rebase-* &&
 	git reset --hard &&