Make 'stg pull' use git-fetch and not git-pull.

We introduce a new pull-does-rebase setting, as companion to pullcmd.
This allows to use both pullcmd's that need a rebase (like git-fetch)
and pullcmd's that do not (like git-pull).

To be able to rebase, we rely on parent information being available
for the stack.

Signed-off-by: Yann Dirson <ydirson@altern.org>