completion: fast initial completion for config 'remote.*.fetch' value

Refspecs for branches in a remote repository start with 'refs/heads/',
so completing those refspecs with 'git config remote.origin.fetch
<TAB>' always offers 'refs/heads/' first, because that's the unique
part of the possible refspecs.  But it does so only after querying the
remote with 'git ls-remote', which can take a while when the request
goes through some slower network to a remote server.

Don't waste the user's time and offer 'refs/heads/' right away for
'git config remote.origin.fetch <TAB>'.

The reason for putting 'refs/heads/' directly into COMPREPLY instead
of using __gitcomp() is to avoid __gitcomp() adding a trailing space.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1891,6 +1891,10 @@ _git_config ()
 	remote.*.fetch)
 		local remote="${prev#remote.}"
 		remote="${remote%.fetch}"
+		if [ -z "$cur" ]; then
+			COMPREPLY=("refs/heads/")
+			return
+		fi
 		__gitcomp_nl "$(__git_refs_remotes "$remote")"
 		return
 		;;