Allow abbreviations in the first refspec to be merged

Allow abbreviations in the first refspec to be merged

The config item for a refspec side and the ref name that it matches
aren't necessarily character-for-character identical. We actually want
to merge a ref by default if: there is no per-branch config, it is the
found result of looking for the match for the first refspec, and the
first refspec is not a pattern. Beyond that, anything that
get_fetch_map() thinks matches is fine.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>