revset: raise ValueError when calling min or max on empty smartset
min([]) raise a ValueError, we do the same thing in smartset.min() and
smartset.max() for the sake of consistency.
The min/amax test are greatly improved in the process to prevent this familly
of regression
$ 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