rebase: don't abort if we're asked to rebase an empty revset

The documentation says we exit 1 if we have nothing to do, so avoid
breaking that contract when we're passed an empty revset.

This was changed in http://www.selenic.com/hg/rev/a259f7b488ab to
improve the error message; keep the improved message, just not the
abort.

hgext/rebase.py

@@ -222,8 +222,9 b' def rebase(ui, repo, **opts):'
             if revf:
                 rebaseset = scmutil.revrange(repo, revf)
                 if not rebaseset:
-                    raise util.Abort(_('empty "rev" revision set - '
-                                       'nothing to rebase'))
+                    ui.status(_('empty "rev" revision set - '
+                                'nothing to rebase\n'))
+                    return 1
             elif srcf:
                 src = scmutil.revrange(repo, [srcf])
                 if not src:

tests/test-rebase-parameters.t

@@ -80,8 +80,8 b' These fail:'
   [255]
 
   $ hg rebase --rev '1 & !1'
-  abort: empty "rev" revision set - nothing to rebase
-  [255]
+  empty "rev" revision set - nothing to rebase
+  [1]
 
   $ hg rebase --source '1 & !1'
   abort: empty "source" revision set - nothing to rebase