rebase: fix rebase with no common ancestors (issue4446)

The new rebase revset didn't check for the case when there are no common
ancestors. Now it does. The new behavior should be the same as the pre-3.2
behavior. Added a test.

hgext/rebase.py

@@ -274,8 +274,12 b' def rebase(ui, repo, **opts):'
                                 "can't compute rebase set\n"))
                     return 1
                 commonanc = repo.revs('ancestor(%ld, %d)', base, dest).first()
+                if commonanc is not None:
                 rebaseset = repo.revs('(%d::(%ld) - %d)::',
                                       commonanc, base, commonanc)
+                else:
+                    rebaseset = []
+
                 if not rebaseset:
                     # transform to list because smartsets are not comparable to
                     # lists. This should be improved to honor lazyness of

tests/test-rebase-parameters.t

@@ -468,3 +468,17 b' Test --tool parameter:'
   [255]
 
   $ cd ..
+
+No common ancestor
+
+  $ hg init separaterepo
+  $ cd separaterepo
+  $ touch a
+  $ hg commit -Aqm a
+  $ hg up -q null
+  $ touch b
+  $ hg commit -Aqm b
+  $ hg rebase -d 0
+  nothing to rebase from d7486e00c6f1 to 3903775176ed
+  [1]
+  $ cd ..