magit-read-stash: actually use PROMPT argument
stash: by default reinstate index when applying
magit-stash-branch: new command
magit-stash-popup: rearrange actions
magit-stash-as-refname: new function
magit-read-stash-history: remove variable

Reading the description in `magit-stash' does not really need
a history variable different from that of other callers of
`read-string'.  In some cases that might even be undesirable.
Merge branch 'n/stash-popup-1' into next