diff: ignore unmerged paths outside prefix with --relative --cached

A diff using --relative ignores entries outside the current directory.
This results in a segfault when we try to process an unmerged entry
that's outside of our prefix, since we end up with a NULL diff_filepair
and use it without checking that it's valid.

I think this bug goes back to 76399c0195 (diff.c: return filepair from
diff_unmerge(), 2011-04-22). Prior to that, diff_unmerge() knew to skip
entries outside of our prefix, due to cd676a5136 (diff --relative:
output paths as relative to the current subdirectory, 2008-02-12). Back
then the caller didn't care that we hadn't added anything to the queue.
In 76399c0195 that changed; we now returned the pair (or NULL), and the
caller in do_oneway_diff() was then called fill_filespec() itself. And
it does so without checking for NULL, causing a segfault.

The obvious fix is to skip the fill_filespec() call (after which we just
return), which this patch does.

There's another call to diff_unmerge() in run_diff_files(). That case
was already fixed by 8174627b3d (diff-lib: ignore paths that are outside
$cwd if --relative asked, 2021-08-22), but of course it didn't help us
for --cached.

That commit also claims that checking the result of diff_unmerge() is
not enough, as we'd want other code paths to skip the entry, too (even
if they wouldn't segfault). But as far as I can tell, that is not true
for --cached. We eventually end up in diff_queue_addremove() or in
diff_queue_change(), both of which know to return early when we're
outside of the prefix.

Arguably we could be checking at the top of oneway_diff() whether the
path is interesting at all. That would not only avoid this code path
entirely, but would also possibly save a small amount of work. But since
everything else appears to work OK, I went for the smallest fix here to
avoid any regression.

Specifically, a comment in oneway_diff() claims we're supposed to
advance o->pos, which we might fail to do if we return early. Though
that "advance" seems to have gone away in da165f470e (unpack-trees.c:
prepare for looking ahead in the index, 2010-01-07), so it is possible
the comment is simply out of date.  We can explore that separately;
checking for a NULL return from diff_unmerge() seems like a sensible
thing to do regardless.

We can piggy-back on the tests added by 8174627b3d; we're just checking
the --cached variant.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>