magit.el: remove unnecessary nesting
magit-git-success: move definition
magit-git-failure: new function
Revert "`magit-ref-ambiguous-p': use `magit-git-success'"

This reverts commit bd45d184e6553686abbdd6ecc9c5a17b311a232c.
magit-ref-ambiguous-p: improve comment

... so that the code doesn't get "fixed" again.
magit-git-{exit-code,string,insert,lines}: flatten ARGS

Flatten the first level of ARGS, the arguments for git.
We already do the same for `magit-{run,call}git' and
the section insert macros.