add unit tests for git_buf_join corner cases

tests/core/buffer.c

@@ -406,6 +406,23 @@ check_joinbuf_2(
 }
 
 static void
+check_joinbuf_overlapped(
+	const char *oldval,
+	int ofs_a,
+	const char *b,
+	const char *expected)
+{
+	char sep = '/';
+	git_buf buf = GIT_BUF_INIT;
+
+	git_buf_sets(&buf, oldval);
+	git_buf_join(&buf, sep, buf.ptr + ofs_a, b);
+	cl_assert(git_buf_oom(&buf) == 0);
+	cl_assert_equal_s(expected, git_buf_cstr(&buf));
+	git_buf_free(&buf);
+}
+
+static void
 check_joinbuf_n_2(
 	const char *a,
 	const char *b,
@@ -480,6 +497,20 @@ void test_core_buffer__8(void)
 	check_joinbuf_2("/abcd/", "defg/", "/abcd/defg/");
 	check_joinbuf_2("/abcd/", "/defg/", "/abcd/defg/");
 
+	check_joinbuf_overlapped("abcd", 0, "efg", "abcd/efg");
+	check_joinbuf_overlapped("abcd", 1, "efg", "bcd/efg");
+	check_joinbuf_overlapped("abcd", 2, "efg", "cd/efg");
+	check_joinbuf_overlapped("abcd", 3, "efg", "d/efg");
+	check_joinbuf_overlapped("abcd", 4, "efg", "efg");
+	check_joinbuf_overlapped("abc/", 2, "efg", "c/efg");
+	check_joinbuf_overlapped("abc/", 3, "efg", "/efg");
+	check_joinbuf_overlapped("abc/", 4, "efg", "efg");
+	check_joinbuf_overlapped("abcd", 3, "", "d/");
+	check_joinbuf_overlapped("abcd", 4, "", "");
+	check_joinbuf_overlapped("abc/", 2, "", "c/");
+	check_joinbuf_overlapped("abc/", 3, "", "/");
+	check_joinbuf_overlapped("abc/", 4, "", "");
+
 	check_joinbuf_n_2("", "", "");
 	check_joinbuf_n_2("", "a", "a");
 	check_joinbuf_n_2("", "/a", "/a");