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).

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.

Allow custom filters with wildcard attributes, so that clients
can support some random `filter=foo` in a .gitattributes and look
up the corresponding smudge/clean commands in the configuration file.

Stash apply: stage new files even when not updating the index