completion: fix bugs with slashes in remote names

completion: fix bugs with slashes in remote names

Previously, some calls to for-each-ref passed fixed numbers of path
components to strip from refs, assuming that remote names had no slashes
in them. This made completions like:

git push github/dseomn :com<Tab>

Result in:

git push github/dseomn :dseomn/completion-remote-slash

With this patch, it instead results in:

git push github/dseomn :completion-remote-slash

Signed-off-by: David Mandelberg <david@mandelberg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>