magit-read-stash: actually use PROMPT argument

(cherry picked from commit f9739c5d680053de831fc2bbec9b21ab90d24217)

Conflicts:
	magit.el
Merge branch 'master' into next

We are "merging" a change that was cherry-picked from `next'
to `master'.  So pick our side, making this an empty commit.

Conflicts:
	magit.el