dir: fix off-by-one error in match_pathspec_item

dir: fix off-by-one error in match_pathspec_item

For a pathspec like 'foo/bar' comparing against a path named "foo/",
namelen will be 4, and match[namelen] will be 'b'.  The correct location
of the directory separator is namelen-1.

However, other callers of match_pathspec_item() such as builtin/grep.c's
submodule_path_match() will compare against a path named "foo" instead of
"foo/".  It might be better to change all the callers to be consistent,
as discussed at
   https://public-inbox.org/git/xmqq7e6cdnkr.fsf@gitster-ct.c.googlers.com/
and
   https://public-inbox.org/git/CABPp-BERWUPCPq-9fVW1LNocqkrfsoF4BPj3gJd9+En43vEkTQ@mail.gmail.com/
but there are many cases to audit, so for now just make sure we handle
both cases with and without a trailing slash.

The reason the code worked despite this sometimes-off-by-one error was
that the subsequent code immediately checked whether the first matchlen
characters matched (which they do) and then bailed and return
MATCHED_RECURSIVELY anyway since wildmatch doesn't have the ability to
check if "name" can be matched as a directory (or prefix) against the
pathspec.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>