rebase: allow non-head rebase-set when obsolete is enabled

Obsolescence markers can represent this situation just fine. Rebased
revisions are marked as precursors of the ones create by
rebase. Unrebased descendants becomes "unstable".

If obsolescence is not enabled we keep the current behavior of
aborting. This new behavior only applies when obsolete is
enabled and is subject to future discussion and changes.

hgext/rebase.py

@@ -194,8 +194,9 b' def rebase(ui, repo, **opts):'
             if not rebaseset:
                 repo.ui.debug('base is ancestor of destination\n')
                 result = None
-            elif not keepf and repo.revs('first(children(%ld) - %ld)-hidden()',
-                                         rebaseset, rebaseset):
+            elif (not (keepf or obsolete._enabled)
+                  and repo.revs('first(children(%ld) - %ld)-hidden()',
+                                rebaseset, rebaseset)):
                 raise util.Abort(
                     _("can't remove original changesets with"
                       " unrebased descendants"),

tests/test-rebase-obsolete.t

@@ -279,4 +279,30 b' not be rebased.'
   32af7686d403cf45b5d95f2d70cebea587ac806a cf44d2f5a9f4297a62be94cbdd3dff7c7dc54258 0 {'date': '*', 'user': 'test'} (glob)
   42ccdea3bb16d28e1848c95fe2e44c000f3f21b1 7c6027df6a99d93f461868e5433f63bde20b6dfb 0 {'date': '*', 'user': 'test'} (glob)
 
-  $ cd ..
+Test ui.prevent-unstable option
+------------------------------------
+
+  $ hg log -r 'children(8)'
+  9:cf44d2f5a9f4 D (no-eol)
+  $ hg rebase -r 8
+  $ hg log -G
+  @  11:0d8f238b634c C
+  |
+  o  10:7c6027df6a99 B
+  |
+  | o  9:cf44d2f5a9f4 D
+  | |
+  | x  8:e273c5e7d2d2 C
+  | |
+  o |  7:02de42196ebe H
+  | |
+  | o  6:eea13746799a G
+  |/|
+  o |  5:24b6387c8c8c F
+  | |
+  | o  4:9520eea781bc E
+  |/
+  o  0:cd010b8cd998 A
+  
+
+