match_explicit: hoist refspec lhs checks into their own function

In preparation for being able to check the left-hand side of
our push refspecs separately, this pulls the examination of
them out into its own function. There should be no behavior
change.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>