fix: suggest --source instead of --rev on empty revset
`--source` is the recommended flag for regular users (`--rev` is
available for advanced users).
Differential Revision: https://phab.mercurial-scm.org/D9224
https://bz.mercurial-scm.org/1089
$ hg init
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..