This change adds `NewCredentialSSHKeyFromSigner`, which allows idiomatic
use of SSH keys from Go. This also lets us spin off an SSH server in the
tests.
This change adds a `go.mod` file. An empty file is sufficient since this
project has no external dependencies. For people that want to use the static
version of libgit2, this module can be vendored and the following can be added
to their `go.mod` file:
replace github.com/libgit2/git2go => ./vendor/github.com/libgit2/git2go