hg: make "_outgoing()" return empty list instead of "None"
This patch makes "_outgoing()" return empty list instead of "None", if
there are no outgoing changesets, because:
- returning "None" requires callers to examine whether returned
value is "None" or not explicitly, if callers want to execute loop
on returned value, but
- there are no explicit needs to return "None"
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown