completion: fill COMPREPLY directly when completing fetch refspecs

completion: fill COMPREPLY directly when completing fetch refspecs

The __git_complete_fetch_refspecs() has to iterate over __git_refs()'s
output anyway to turn the listed matching refs into refspecs, and it
knows about the prefix and suffix that has to be added to each
refspec.

Modify this function to add the prefix and suffix to each refspec
while iterating and feed the result, since it doesn't need further
processing, to the new __gitcomp_direct() helper added in the previous
commit, because it should be faster when there are a lot of refspecs
to list.

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