stash: be careful what we store

stash: be careful what we store

"git stash store" is meant to store what "git stash create"
produces, as these two are implementation details of the end-user
facing "git stash save" command.  Even though it is clearly
documented as such, users would try silly things like "git stash
store HEAD" to render their stash unusable.

Worse yet, because "git stash drop" does not allow such a stash
entry to be removed, "git stash clear" would be the only way to
recover from such a mishap.  Reuse the logic that allows "drop" to
refrain from working on such a stash entry to teach "store" to avoid
storing an object that is not a stash entry in the first place.

Signed-off-by: Junio C Hamano <gitster@pobox.com>