Update reflog message created for stashes

Update reflog message created for stashes

A stash is about a change on top of an existing commit, and not
about that commit that happened to be on which the change was
created.  Match the message we see in "git stash list" with the
commit log message to make this clear.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Uwe Kleine-König <ukleinek@informatik.uni-freiburg.de>