diff: add failing test for racy-git in the index

We update the index and then immediately change the contents of the
file. This makes the diff think there are no changes, as the timestamp
of the file agrees with the cached data. This is however a bug, as the
file has obviously changed contents.

The test is a bit fragile, as it assumes that the index writing and the
following modification of the file happen in the same second, but it's
enough to show the issue.

tests/diff/racy.c

+#include "clar_libgit2.h"
+
+#include "buffer.h"
+
+static git_repository *g_repo;
+
+void test_diff_racy__initialize(void)
+{
+	cl_git_pass(git_repository_init(&g_repo, "diff_racy", false));
+}
+
+void test_diff_racy__cleanup(void)
+{
+	cl_git_sandbox_cleanup();
+}
+
+void test_diff_racy__diff(void)
+{
+	git_index *index;
+	git_diff *diff;
+	git_buf path = GIT_BUF_INIT;
+
+	cl_git_pass(git_buf_joinpath(&path, git_repository_workdir(g_repo), "A"));
+	cl_git_mkfile(path.ptr, "A");
+
+	/* Put 'A' into the index */
+	cl_git_pass(git_repository_index(&index, g_repo));
+	cl_git_pass(git_index_add_bypath(index, "A"));
+	cl_git_pass(git_index_write(index));
+
+	cl_git_pass(git_diff_index_to_workdir(&diff, g_repo, index, NULL));
+	cl_assert_equal_i(0, git_diff_num_deltas(diff));
+
+	/* Change its contents quickly, so we get the same timestamp */
+	cl_git_mkfile(path.ptr, "B");
+
+	cl_git_pass(git_diff_index_to_workdir(&diff, g_repo, index, NULL));
+	cl_assert_equal_i(1, git_diff_num_deltas(diff));
+}