stash: call reflog_delete() in reflog.c

stash: call reflog_delete() in reflog.c

Now that cmd_reflog_delete has been libified an exported it into a new
reflog.c library so we can call it directly from builtin/stash.c. This
not only gives us a performance gain since we don't need to create a
subprocess, but it also allows us to use the ref transactions api in the
future.

Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>