zsh completion: find matching custom bash completion

If zsh completion is being read from a location that is different from
system-wide default, it is likely that the user is trying to use a
custom version, perhaps closer to the bleeding edge, installed in her
own directory. We will more likely to find the matching bash completion
script in the same directory than in those system default places.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
completion: introduce __gitcomp_nl_append ()

There are situations where multiple classes of completions possible. For
example

  branch.<TAB>

should try to complete

  branch.master.
  branch.autosetupmerge
  branch.autosetuprebase

The first candidate has the suffix ".", and the second/ third candidates
have the suffix " ". To facilitate completions of this kind, create a
variation of __gitcomp_nl () that appends to the existing list of
completion candidates, COMPREPLY.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
completion: fix branch.autosetup(merge|rebase)

When attempting to complete

  $ git config branch.auto<TAB>

'autosetupmerge' and 'autosetuprebase' don't come up. This is because
"$cur" is matched with "branch.*" and a list of branches are
completed. Add 'autosetupmerge', 'autosetuprebase' as candidates for
completion too, using __gitcomp_nl_append ().

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
completion: fix remote.pushdefault

When attempting to complete

  $ git config remote.push<TAB>

'pushdefault' doesn't come up. This is because "$cur" is matched with
"remote.*" and a list of remotes are completed. Add 'pushdefault' as a
candidate for completion too, using __gitcomp_nl_append ().

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>