scripts/feeds: Support feed update with git pull --rebase --autostash

scripts/feeds: Support feed update with git pull --rebase --autostash

Add feed update option '-s' that performs 'git pull --rebase --autostash':
* possible local uncommited changes are autostashed before pull,
* local commits are then rebased on top of the new commits pulled
  from origin and
* finally git does 'stash pop'.

This enables feed update while there are local development commits
and possibly also local uncommited changes.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
Link: https://github.com/openwrt/openwrt/pull/15377
Signed-off-by: Robert Marko <robimarko@gmail.com>