completion: expand "push --delete <remote> <ref>" for refs on that <remote>

completion: expand "push --delete <remote> <ref>" for refs on that <remote>

Change the completion of "push --delete <remote> <ref>" to complete
refs on that <remote>, not all refs.

Before this cloning git.git and doing "git push --delete origin
p<TAB>" will complete nothing, since a fresh clone of git.git will
have no "pu" branch, whereas origin/p<TAB> will uselessly complete
origin/pu, but fully qualified references aren't accepted by
"--delete".

Now p<TAB> will complete as "pu". The completion of giving --delete
later, e.g. "git push origin --delete p<TAB>" remains unchanged, this
is a bug, but is a general existing limitation of the bash completion,
and not how git-push is documented, so I'm not fixing that case, but
adding a failing TODO test for it.

The testing code was supplied by SZEDER Gábor in
<20170421122832.24617-1-szeder.dev@gmail.com> with minor setup
modifications on my part.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: SZEDER Gábor <szeder.dev@gmail.com>
Test-code-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>