patchbomb: treat empty address list as no addresses
Previously it wasn't possible to use configuration to avoid
being prompted for e.g. a CC list when using patchbomb to send
emails.
We now make it possible to supply an empty value.
$ 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