rebase: better way to detect non-detaching revisions (issue5044)

Rather than look for the lowest revision, see if the rebase state is tracking
the parents of this revision. Otherwise we can't handle multiple revisions in
one rebase that includes a merge revision.

Fixes issue5044.

hgext/rebase.py

@@ -712,8 +712,8 b' def defineparents(repo, rev, target, sta'
     repo.ui.debug(" future parents are %d and %d\n" %
                             (repo[p1].rev(), repo[p2].rev()))
 
-    if rev == min(state):
-        # Case (1) initial changeset of a non-detaching rebase.
+    if not any(p.rev() in state for p in parents):
+        # Case (1) root changeset of a non-detaching rebase set.
         # Let the merge mechanism find the base itself.
         base = None
     elif not repo[rev].p2():

tests/test-rebase-parameters.t

@@ -412,6 +412,16 b' Specify only revs (from 2 onto 8)'
   
   $ cd ..
 
+Rebasing both a single revision and a merge in one command
+
+  $ hg clone -q -u . a aX
+  $ cd aX
+  $ hg rebase -r 3 -r 6
+  rebasing 3:32af7686d403 "D"
+  rebasing 6:eea13746799a "G"
+  saved backup bundle to $TESTTMP/aX/.hg/strip-backup/eea13746799a-ad273fd6-backup.hg (glob)
+  $ cd ..
+
 Test --tool parameter:
 
   $ hg init b