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
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls -A
.hg
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls -A
.hg
a