attr: more matching optimizations from .gitignore

.gitattributes and .gitignore share the same pattern syntax but has
separate matching implementation. Over the years, ignore's
implementation accumulates more optimizations while attr's stays the
same.

This patch reuses the core matching functions that are also used by
excluded_from_list. excluded_from_list and path_matches can't be
merged due to differences in exclude and attr, for example:

* "!pattern" syntax is forbidden in .gitattributes.  As an attribute
  can be unset (i.e. set to a special value "false") or made back to
  unspecified (i.e. not even set to "false"), "!pattern attr" is unclear
  which one it means.

* we support attaching attributes to directories, but git-core
  internally does not currently make use of attributes on
  directories.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitattributes.txt

@@ -56,6 +56,7 @@ When more than one pattern matches the path, a later line
 overrides an earlier line.  This overriding is done per
 attribute.  The rules how the pattern matches paths are the
 same as in `.gitignore` files; see linkgit:gitignore[5].
+Unlike `.gitignore`, negative patterns are forbidden.
 
 When deciding what attributes are assigned to a path, git
 consults `$GIT_DIR/info/attributes` file (which has the highest