filebuf: failing test for leaving the lockfile when failing to rename

When we fail to rename, we currently leave the lockfile laying
around. This shows that behaviour.

tests/core/filebuf.c

@@ -124,3 +124,30 @@ void test_core_filebuf__umask(void)
 	cl_must_pass(p_unlink(test));
 }
 
+void test_core_filebuf__rename_error(void)
+{
+	git_filebuf file = GIT_FILEBUF_INIT;
+	char *dir = "subdir",  *test = "subdir/test", *test_lock = "subdir/test.lock";
+	int fd;
+
+#ifndef GIT_WIN32
+	cl_skip();
+#endif
+
+	cl_git_pass(p_mkdir(dir, 0666));
+	cl_git_mkfile(test, "dummy content");
+	fd = p_open(test, O_RDONLY);
+	cl_assert(fd > 0);
+	cl_git_pass(git_filebuf_open(&file, test, 0, 0666));
+
+	cl_git_pass(git_filebuf_printf(&file, "%s\n", "libgit2 rocks"));
+
+	cl_assert_equal_i(true, git_path_exists(test_lock));
+
+	cl_git_fail(git_filebuf_commit(&file));
+	p_close(fd);
+
+	git_filebuf_cleanup(&file);
+	cl_assert_equal_i(false, git_path_exists(test_lock));
+
+}