Accept the "HOST/OWNER/REPO" syntax or passing a full URL for both the
`--repo` flag and the GH_REPO environment variable and allow setting
GH_HOST environment variable to override just the hostname for
operations that assume "github.com" by default.
Examples:
$ gh repo clone example.org/owner/repo
$ GH_HOST=example.org gh repo clone repo
$ GH_HOST=example.org gh api user
$ GH_HOST=example.org gh gist create myfile.txt
$ gh issue list -R example.org/owner/repo
$ gh issue list -R https://example.org/owner/repo.git
$ GH_REPO=example.org/owner/repo gh issue list
This commit hacks the existing repo clone tests into something usable by
the new isolated command. It went ok and was less effort than trying to
introduce the same kind of test format as repo view and gist create.
