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 is another isolation refactor, this time for repo fork.
However, I got fed up with the --remote="true|false|prompt" style of
flags and took this opportunity to switch to a set of bool flags:
--remote and --clone
--no-remote and --no-clone
the string args were really non standard and confusing; with only two
bools it was impossible to tell when to prompt.
