Supporting restarting a mgw session.

Normally this happens post rebase and before mgw would disallow if
local master was not on the same commit as origin/master. Now we allow
it through so long as origin/master can be fast-forward merged into
master.
Implementing a merge continue and rebase continue. Needs the changes from feature/mgw_detectancestor_push to work
Merge branch 'feature/mgw_detectancestor_push' into feature/mgw_continue_rebasemerge

Conflicts:
	lib/EnsEMBL/Git.pm