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.
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