support autostash for pull rebase

**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

Trace GitUI's pull and rebase entry points, then inspect how Git configuration is handled. Verify the chosen behavior with the existing pull/rebase coverage, and consider the work done when pulling with local changes supports --autostash or respects rebase.autoStash.

Written by the indexing model from the issue text.