dir: match "attr" pathspec magic with correct paths

dir: match "attr" pathspec magic with correct paths

The match_pathspec_item() function takes "prefix" value, allowing a
caller to chop off the common leading prefix of pathspec pattern
strings from the path and only use the remainder of the path to
match the pathspec patterns (after chopping the same leading prefix
of them, of course).

This "common leading prefix" optimization has two main features:

 * discard the entries in the in-core index that are outside of the
   common leading prefix; if you are doing "ls-files one/a one/b",
   we know all matches must be from "one/", so first the code
   discards all entries outside the "one/" directory from the
   in-core index.  This allows us to work on a smaller dataset.

 * allow skipping the comparison of the leading bytes when matching
   pathspec with path.  When "ls-files" finds the path "one/a/1" in
   the in-core index given "one/a" and "one/b" as the pathspec,
   knowing that common leading prefix "one/" was found lets the
   pathspec matchinery not to bother comparing "one/" part, and
   allows it to feed "a/1" down, as long as the pathspec element
   "one/a" gets corresponding adjustment to "a".

When the "attr" pathspec magic is in effect, however, the current
code breaks down.

The attributes, other than the ones that are built-in and the ones
that come from the $GIT_DIR/info/attributes file and the top-level
.gitattributes file, are lazily read from the filesystem on-demand,
as we encounter each path and ask if it matches the pathspec.  For
example, if you say "git ls-files "(attr:label)sub/" in a repository
with a file "sub/file" that is given the 'label' attribute in
"sub/.gitattributes":

 * The common prefix optimization finds that "sub/" is the common
   prefix and prunes the in-core index so that it has only entries
   inside that directory.  This is desirable.

 * The code then walks the in-core index, finds "sub/file", and
   eventually asks do_match_pathspec() if it matches the given
   pathspec.

 * do_match_pathspec() calls match_pathspec_item() _after_ stripping
   the common prefix "sub/" from the path, giving it "file", plus
   the length of the common prefix (4-bytes), so that the pathspec
   element "(attr:label)sub/" can be treated as if it were "(attr:label)".

The last one is what breaks the match in the current code, as the
pathspec subsystem ends up asking the attribute subsystem to find
the attribute attached to the path "file".  We need to ask about the
attributes on "sub/file" when calling match_pathspec_attrs(); this
can be done by looking at "prefix" bytes before the beginning of
"name", which is the same trick already used by another piece of the
code in the same match_pathspec_item() function.

Unfortunately this was not discovered so far because the code works
with slightly different arguments, e.g.

 $ git ls-files "(attr:label)sub"
 $ git ls-files "(attr:label)sub/" "no/such/dir/"

would have reported "sub/file" as a path with the 'label' attribute
just fine, because neither would trigger the common prefix
optimization.

Reported-by: Matthew Hughes <mhughes@uw.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>