As with the callbacks, third-party implementations of smart subtransports cannot
reach into the opaque struct and thus cannot know what options the user set.

Add a getter for these options to copy the proxy options into something external
implementors can use.

Added `git_repository_submodule_cache_all` to initialze a cache of
submodules on the repository so that operations looking up N
submodules are O(N) and not O(N^2).  Added a
`git_repository_submodule_cache_clear` function to remove the cache.

Also optimized the function that loads all submodules as it was itself
O(N^2) w.r.t the number of submodules, having to loop through the
`.gitmodules` file once per submodule.  I changed it to process the
`.gitmodules` file once, into a map.

Signed-off-by: David Turner <dturner@twosigma.com>

When writing an object, we calculate its OID and see if it exists in the
object database.  If it does, we need to freshen the file that contains
it.

I don't quite recall what we do in the other places where we use this,
but we should pass this payload to the callbacks.