The function `git_attr_cache__insert_macro` is responsible for adopting
macros in the per-repo macro cache. When adding a macro that replaces an
already existing macro (e.g. because of re-parsing gitattributes files),
then we do not free the previous macro and thus cause a memory leak.

Fix this leak by first checking if the cache already has a macro defined
with the same name. If so, free it before replacing the cache entry with
the new instance.