rebase: improve error message for empty --source set
Before, it just said 'nothing to rebase' in this case. Now, it aborts
mentioning the reason: 'empty "source" revision set'.
Specifying revisions that cannot be rebased is a 'soft' error, but specifying
an empty set deserves an abort that explains exactly what the problem is.
$ "$TESTDIR/hghave" execbit || exit 80
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..