In libgit2 nomenclature, when we need to verb a direct object, we name
a function `git_directobject_verb`.  Thus, if we need to init an options
structure named `git_foo_options`, then the name of the function that
does that should be `git_foo_options_init`.

The previous names of `git_foo_init_options` is close - it _sounds_ as
if it's initializing the options of a `foo`, but in fact
`git_foo_options` is its own noun that should be respected.

Deprecate the old names; they'll now call directly to the new ones.

The trace logging callbacks should match the other callback naming
conventions, using the `_cb` suffix instead of a `_callback` suffix.

The credential callbacks should match the other callback naming
conventions, using the `_cb` suffix instead of a `_callback` suffix.

For consistency with other "type" enums, rename
git_remote_completion_type to git_remote_completion_t.

Safely deprecate `git_push_transfer_progress`, forwarding it to the new
`git_push_transfer_progress_cb` name.

Safely deprecate `git_transfer_progress` and `git_transfer_progress_cb`
types, forwarding them to the new `git_indexer_progress` and
`git_indexer_progress_cb`.

Users can define `GIT_DEPRECATE_HARD` if they want to remove all
functions that we've "softly" deprecated.

Add `@deprecated` to the functions that are, so that they'll appear that
way in docurium.