This updates to implementation of gitattribute macros to be much more
similar to core git (albeit not 100%) and to handle expansion of
macros within macros, etc.  It also cleans up the refcounting usage
with macros to be much cleaner.

Also, this adds a new vector function `git_vector_insert_sorted()`
which allows you to maintain a sorted list as you go.  In order to
write that function, this changes the function `git__bsearch()` to
take a somewhat different set of parameters, although the core
functionality is still the same.