git-pull.sh: introduce --[no-]autostash and pull.autostash

This new configuration variable executes 'git stash' before attempting
to merge/rebase, and 'git stash pop' after a successful merge/rebase.
It makes it convenient for people to pull with dirty worktrees.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>