completion: support completing fully qualified non-fast-forward refspecs

completion: support completing fully qualified non-fast-forward refspecs

After 'git fetch <remote> <TAB>' our completion script offers refspecs
that will fetch to a local branch with the same name as in the remote
repository, e.g. 'master:master'.  This also completes
non-fast-forward refspecs, i.e. after a '+' prefix like
'+master:master', and fully qualified refspecs, e.g.
'refs/heads/master:refs/heads/master'.  However, it does not complete
non-fast-forward fully qualified refspecs (or fully qualified refspecs
following any other prefix, e.g. '--option=', though currently no git
command supports such an option, but third party git commands might).

These refspecs are listed by the __git_refs2() function, which is just
a thin wrapper iterating over __git_refs()'s output, turning each
listed ref into a refspec.  Now, it's certainly possible to modify
__git_refs2() and its callsite to pass an extra parameter containing
only the ref part of the current word to be completed (to follow suit
of the previous commit) to deal with prefixed fully qualified refspecs
as well.  Unfortunately, keeping the current behavior unchanged in the
"no extra parameter" case brings in a bit of subtlety, which makes the
resulting code ugly and compelled me to write a 8-line long comment in
the proof of concept.  Not good.  However, since the callsite has to
be modified for proper functioning anyway, we might as well leave
__git_refs2() as is and introduce a new helper function without
backwards compatibility concerns.

Add the new function __git_complete_fetch_refspecs() that has all the
necessary parameters to do the right thing in all cases mentioned
above, including non-fast-forward fully qualified refspecs.  This new
function can also easier benefit from optimizations coming later in
this patch series.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>