tests: fix a relative pathname issue

The path given to `git_index_add_bypath` is relative to the root of the
repository. That `describe/file` path is relative to the root of the
sandbox directory, hence if I add the missing `cl_git_pass` I rightfully
get an error that `$SANDBOX/describe/describe/file doesn't exist`.

The path is thus changed to be made relative to the repository, which
makes the failure go away and "restore" the test.

tests/describe/t6120.c

@@ -104,7 +104,7 @@ static void commit_and_tag(
 
 	cl_git_append2file("describe/file", "\n");
 	
-	cl_git_pass(git_index_add_bypath(index, "describe/file"));
+	cl_git_pass(git_index_add_bypath(index, "file"));
 	cl_git_pass(git_index_write(index));
 
 	*time += 10;