We no longer guess the head repository using heuristics; instead, we
present the user with the choice of pushable repositories and an
additional option to create a new fork.
The new `pr create --head` flag is available for the user to specify the
head branch in `branch` or `owner:branch` format and completely skip any
forking or auto-pushing checks.
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.
