copy: give better error message when no source paths found with --at-rev
The new error message matches what we show when marking copies in the
working copy.
Differential Revision: https://phab.mercurial-scm.org/D8496
$ 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
.hg
a
unknown