We now have `_fromstate` and `_fromstate_on_head`, the latter of which updates
the current branch to point to the new commit.

Currently git_commit_create() takes on creating a commit and updating a
reference. Provide a better interface by splitting up each of the
concerns into named functions for each.

git_commit_create() will only create the commit, it will not modify any
reference. git_commit_create_on() takes a reference name to update and
git_commit_create_on_head() is a convenience function to update HEAD.

This variant of the commit creation function takes the reference to
update, the tree and the parents from the current branch, index and
merging state, allowing for simpler use of a common use-case.

RFC 3986 says that hostnames can be percent encoded.  Percent decode
hostnames in our URLs.

Now that we can decode percent-encoded strings as part of `git_buf`s,
use that decoder in `gitno_extract_url_parts`.

Use `git_buf_decode_percent` so that we can avoid allocating a temporary
buffer.

Introduce a function to take a percent-encoded string (URI encoded,
described by RFC 1738) and decode it into a `git_buf`.