builtin-apply.c: Skip filenames without enough components

builtin-apply.c: Skip filenames without enough components

find_name() wrongly returned the whole filename for filenames without
enough leading pathname components (e.g., when applying a patch to a
top-level file with -p2).

Include the -p value used in the error message when no filenames can be
found.

[jc: squashed a test from Nanako Shiraishi]

Signed-off-by: Andreas Gruenbacher <agruen@suse.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>