checkout: -m (--merge) uses autostash when switching branches

checkout: -m (--merge) uses autostash when switching branches

When switching branches with "git checkout -m", local modifications
can block the switch.  Teach the -m flow to create a temporary stash
before switching and reapply it after.  On success, only "Applied
autostash." is shown.  If reapplying causes conflicts, the stash is
kept and the user is told they can resolve and run "git stash drop",
or run "git reset --hard" and later "git stash pop" to recover their
changes.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>