update gitoxide to v0.21.1 (#752)

* update gitoxide to v0.21.1

The most notable improvement is its ability to open submodule
repositories, thus increasing the amount of repositories onefetch
can handle.

* upgrade `git-path` to latest version

This fixes an issue with submodules not being handled correctly if `.`
was passed as input for discovery.

* avoid instantiating a `git2` repo just to see if it is bare.

Instead, postpone the work-tree check to when it is needed and exit
with the same message.

* try to avoid pulling in sha-1 assembly while leaving everything else unchanged.