It rarely makes sense to push commits with conflicts to a remote Git
repo (which is the only kind of remote we support so far), so let's
just error out instead of pushing a commit that others pulling from
the remote probably can't make sense of.
I've only added a simple test for the error case for now. `libgit2`
doesn't support pushing to a local repo, so it's harder to test the
success case. I suppose we'll have to have the regular `git` binary
running local servers in test eventually.
Closes#60.
