Allow adding a submodule through git_index_add_bypath

refdb: delete a ref's reflog upon deletion

git_fetch_options was missing from the API docs because it lacked a
documentation comment above the struct declaration.
I used the git_checkout_options docstring as a template.

Also fixes a typo in git_remote_prune_refs (remote, not reamote).

Removing a reflog upon ref deletion is something which only some
backends might wish to do. Backends which are database-backed may wish
to archive a reflog, log-based ones may not need to do anything.

This is to be returned when the operation which the user asked for is
not possible to do on a directory.

filters: custom filters with wildcard attributes

Added git_diff_index_to_index()

These functions are available on the public API but don't have any
documentation, so they don't appear on the API reference. Fix that.

The function was removed, but its declaration and changelog entry about
its removal were forgotten.

The comment in the test doesn't make any sense as the function doesn't
exist anymore, so get rid of it as well.