git sync: option to not push

[teumas]

Sometimes when hacking away on a feature branch I would like to perform git sync in order to fetch changes from the remote, but without pushing my changes to the remote.

Something like git sync --no-push, which performs all the steps of sync but stops short of pushing my local changes to the remote branch.

This would be great when I need to verify that my code and the changes from upstream work together correctly before I push my changes.

[kevgo]

related: #1680