var: adjust memory allocation for strings

var: adjust memory allocation for strings

Right now, all of our values are constants whose allocation is managed
elsewhere.  However, in the future, we'll have some variables whose
memory we will need to free.  To keep things consistent, let's make each
of our functions allocate its own memory and make the caller responsible
for freeing it.

Signed-off-by: brian m. carlson <bk2204@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>