support autostash for pull rebase

[xandudex]

Is your feature request related to a problem? Please describe.
Pulling fails when there are uncommitted local changes, forcing a manual stash -> pull -> pop workflow. The rebase.autoStash setting in git config is currently ignored.

Describe the solution you'd like
Support --autostash when pulling/rebasing, or respect the rebase.autoStash git config automatically.

Describe alternatives you've considered

• Manually stashing and popping in GitUI's stash tab.
• Dropping to the terminal to run git pull.

Additional context
There was a closed issue #1399 somehow related to that problem