magit-with-temp-index: new macro
magit-stash-popup: bind magit-stash-index-snapshot as intended
magit-insert-stashes: use log margin

For stashes show the relative age in the log margin, but not the author.
We do so by using `magit-log-wash-log' to wash the list of stashes.
magit-stash{,-index}-snapshot: remove date from message

Now that the log margin supports stashes, we no longer need to
insert the date into the message.  Unfortunately we cannot use

  "Snapshot on <branch>: <hash> <summary>"

which would be like

  "WIP on <branch>: <hash> <summary>"

used when no message is provided, because if a message is
provided then `git stash save' insists on prefixing that with

  "On <branch>: "

So we use

  "On <branch>: " + "Snapshot: <hash> <summary>"