remote.c: add a function for deleting a refspec array and use it (twice)

remote.c: add a function for deleting a refspec array and use it (twice)

A number of call sites allocate memory for a refspec array, populate
its members with heap memory, and then free only the refspec pointer
while leaking the memory allocated for the member elements. Provide
a function for freeing the elements of a refspec array and the array
itself.

Caution to callers: code paths must be checked to ensure that the
refspec members "src" and "dst" can be passed to free.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>