rebase: don't rebase obsolete commits with no successor

This patch avoids unnecessary conflicts to resolve during rebase for the users
of changeset evolution.

This patch modifies rebase to skip obsolete commits with no successor.
It introduces a new rebase state 'revpruned' for these revisions that are
being skipped and a new message to inform the user of what is happening.
This feature is gated behind the config flag experimental.rebaseskipobsolete

When an obsolete commit is skipped, the output is:
note: not rebasing 7:360bbaa7d3ce "O", it has no successor

hgext/rebase.py

@@ -32,6 +32,7 b' nullmerge = -2'
 revignored = -3
 # To do with obsolescence
 revprecursor = -4
+revpruned = -5
 
 cmdtable = {}
 command = cmdutil.command(cmdtable)
@@ -487,6 +488,9 b' def rebase(ui, repo, **opts):'
                              targetctx.description().split('\n', 1)[0])
                 msg = _('note: not rebasing %s, already in destination as %s\n')
                 ui.status(msg % (desc, desctarget))
+            elif state[rev] == revpruned:
+                msg = _('note: not rebasing %s, it has no successor\n')
+                ui.status(msg % desc)
             else:
                 ui.status(_('already rebased %s as %s\n') %
                           (desc, repo[state[rev]]))
@@ -676,7 +680,7 b' def defineparents(repo, rev, target, sta'
     elif p1n in state:
         if state[p1n] == nullmerge:
             p1 = target
-        elif state[p1n] in (revignored, revprecursor):
+        elif state[p1n] in (revignored, revprecursor, revpruned):
             p1 = nearestrebased(repo, p1n, state)
             if p1 is None:
                 p1 = target
@@ -692,7 +696,7 b' def defineparents(repo, rev, target, sta'
         if p2n in state:
             if p1 == target: # p1n in targetancestors or external
                 p1 = state[p2n]
-            elif state[p2n] in (revignored, revprecursor):
+            elif state[p2n] in (revignored, revprecursor, revpruned):
                 p2 = nearestrebased(repo, p2n, state)
                 if p2 is None:
                     # no ancestors rebased yet, detach
@@ -882,7 +886,7 b' def restorestatus(repo):'
             else:
                 oldrev, newrev = l.split(':')
                 if newrev in (str(nullmerge), str(revignored),
-                              str(revprecursor)):
+                              str(revprecursor), str(revpruned)):
                     state[repo[oldrev].rev()] = int(newrev)
                 elif newrev == nullid:
                     state[repo[oldrev].rev()] = revtodo
@@ -1066,7 +1070,10 b' def buildstate(repo, dest, rebaseset, co'
         for ignored in set(rebasedomain) - set(rebaseset):
             state[ignored] = revignored
     for r in obsoletenotrebased:
-        state[r] = revprecursor
+        if obsoletenotrebased[r] is None:
+            state[r] = revpruned
+        else:
+            state[r] = revprecursor
     return repo['.'].rev(), dest.rev(), state
 
 def clearrebased(ui, repo, state, skipped, collapsedas=None):
@@ -1180,7 +1187,9 b' def _rebasedvisible(orig, repo):'
 
 def _computeobsoletenotrebased(repo, rebasesetrevs, dest):
     """return a mapping obsolete => successor for all obsolete nodes to be
-    rebased that have a successors in the destination"""
+    rebased that have a successors in the destination
+
+    obsolete => None entries in the mapping indicate nodes with no succesor"""
     obsoletenotrebased = {}
 
     # Build a mapping successor => obsolete nodes for the obsolete
@@ -1206,6 +1215,11 b' def _computeobsoletenotrebased(repo, reb'
         for s in allsuccessors:
             if s in ancs:
                 obsoletenotrebased[allsuccessors[s]] = s
+            elif (s == allsuccessors[s] and
+                  allsuccessors.values().count(s) == 1):
+                # plain prune
+                obsoletenotrebased[s] = None
+
     return obsoletenotrebased
 
 def summaryhook(ui, repo):

tests/test-rebase-obsolete.t

@@ -659,3 +659,29 b' Even when the chain include missing node'
   $ hg rebase -d 'desc(B2)'
   note: not rebasing 1:a8b11f55fb19 "B0", already in destination as 2:261e70097290 "B2"
   rebasing 5:1a79b7535141 "D" (tip)
+  $ hg up 4
+  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ echo "O" > O
+  $ hg add O
+  $ hg commit -m O
+  $ echo "P" > P
+  $ hg add P
+  $ hg commit -m P
+  $ hg log -G
+  @  8:8d47583e023f P
+  |
+  o  7:360bbaa7d3ce O
+  |
+  | o  6:9c48361117de D
+  | |
+  o |  4:ff2c4d47b71d C
+  |/
+  o  2:261e70097290 B2
+  |
+  o  0:4a2df7238c3b A
+  
+  $ hg debugobsolete `hg log -r 7 -T '{node}\n'` --config experimental.evolution=all
+  $ hg rebase -d 6 -r "4::"
+  rebasing 4:ff2c4d47b71d "C"
+  note: not rebasing 7:360bbaa7d3ce "O", it has no successor
+  rebasing 8:8d47583e023f "P" (tip)