test: don't do a shallow fetch for signoff check

If the user's PR doesn't have the current lxc/master commit in its history,
this check fails miserably since the shallow fetch won't have whatever patch
their branch is based on. We should instead fetch all the history, so a branch
from any commit checks correctly.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>