Git: Do not pull --rebase when changes are not stashed

Git will not do a pull --rebase when there are changes. So we
now abort if the user did not stash them instead of trying to
pull.

I do not want to revert user changes implicitly.

Task-number: QTCREATORBUG-7031
Change-Id: Ic6041755938be36d8ecdbcbac373b5c38642360d
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>