`magit-ref-ambiguous-p': use `magit-git-success'

Since commit f8d304a1, `magit-ref-ambiguous-p' always returned nil.

Instead of depending on how one of our functions behaves when presented
with `git-rev-parse(1)'s error messages, just check its exit code...

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #933 from praet/git-string-victim

fix last `magit-git-string' victim