Explicitly specify date format for magit-insert-stashes

If Git configuration option log.date is set to non-default value (for
example iso8601), then "git stash list" output will be like

stash@{2013-12-03 16:13:51 +0200}: WIP on master: 7ae906e ...

This breaks the parsing of the output, so use --date=default to make the
stash list output in correct format, e.g.:

stash@{0}: WIP on master: 7ae906e ...