rebase -r: demonstrate bug with conflicting merges

When calling `merge` on a branch that has already been merged, that
`merge` is skipped quietly, but currently a MERGE_HEAD file is being
left behind and will then be grabbed by the next `pick` (that did
not want to create a *merge* commit).

Demonstrate this.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3430-rebase-merges.sh

@@ -396,4 +396,20 @@ test_expect_success 'with --autosquash and --exec' '
 	grep "G: +G" actual
 '
 
+test_expect_failure '--continue after resolving conflicts after a merge' '
+	git checkout -b already-has-g E &&
+	git cherry-pick E..G &&
+	test_commit H2 &&
+
+	git checkout -b conflicts-in-merge H &&
+	test_commit H2 H2.t conflicts H2-conflict &&
+	test_must_fail git rebase -r already-has-g &&
+	grep conflicts H2.t &&
+	echo resolved >H2.t &&
+	git add -u &&
+	git rebase --continue &&
+	test_must_fail git rev-parse --verify HEAD^2 &&
+	test_path_is_missing .git/MERGE_HEAD
+'
+
 test_done