Clarify the clone and with subcommands.

clone should be capable of taking in any kind of Git URI, be it a
local directory path, or a URI like git://github.com/Oblomov/zit.git
or git@github.com:Oblomov/zit.git

with doesn't actually run git directly, it just passes commands as-is
after setting up the environment. This creates some awkwardness as you
need to type out things like "zit with zit git diff", but there are a
few git* commands that don't conform to the "git SUBCOMMAND" syntax,
too, or even commands external from core Git.