Currently, the `git__free` function is being defined in a single place,
only, disregarding whether we use our standard allocators or the crtdbg
allocators. This makes it a bit harder to convert our code base to use
pluggable allocators, and furthermore makes the border between our two
allocators a bit more blurry.

Implement a separate `git__crtdbg__free` function for the crtdbg
allocator in order to completely separate both allocator
implementations.