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.

hgext/rebase.py

@@ -226,7 +226,11 b' def rebase(ui, repo, **opts):'
                                        'nothing to rebase'))
             elif srcf:
                 src = scmutil.revrange(repo, [srcf])
+                if not src:
+                    raise util.Abort(_('empty "source" revision set - '
+                                       'nothing to rebase'))
                 rebaseset = repo.revs('(%ld)::', src)
+                assert rebaseset
             else:
                 base = scmutil.revrange(repo, [basef or '.'])
                 rebaseset = repo.revs(

tests/test-rebase-parameters.t

@@ -83,6 +83,10 b' These fail:'
   abort: empty "rev" revision set - nothing to rebase
   [255]
 
+  $ hg rebase --source '1 & !1'
+  abort: empty "source" revision set - nothing to rebase
+  [255]
+
   $ hg rebase
   nothing to rebase
   [1]