builtin-revert.c: release index lock when cherry-picking an empty commit

When a cherry-pick of an empty commit is done, release the lock
held on the index.

The fix is the same as was applied to similar code in 4271666.

Signed-off-by: Chris Johnsen <chris_johnsen@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-revert.c

@@ -387,6 +387,7 @@ static int revert_or_cherry_pick(int argc, const char **argv)
 	    (write_cache(index_fd, active_cache, active_nr) ||
 	     commit_locked_index(&index_lock)))
 		die("%s: Unable to write new index file", me);
+	rollback_lock_file(&index_lock);
 
 	if (!clean) {
 		add_to_msg("\nConflicts:\n\n");

t/t3505-cherry-pick-empty.sh

@@ -0,0 +1,33 @@
+#!/bin/sh
+
+test_description='test cherry-picking an empty commit'
+
+. ./test-lib.sh
+
+test_expect_success setup '
+
+	echo first > file1 &&
+	git add file1 &&
+	test_tick &&
+	git commit -m "first" &&
+
+	git checkout -b empty-branch &&
+	test_tick &&
+	git commit --allow-empty -m "empty"
+
+'
+
+test_expect_code 1 'cherry-pick an empty commit' '
+
+	git checkout master &&
+	git cherry-pick empty-branch
+
+'
+
+test_expect_success 'index lockfile was removed' '
+
+	test ! -f .git/index.lock
+
+'
+
+test_done