rebase: block collapse with keepbranches on multiple named branches (issue2112)

Collapse and keepbranches should be blocked when there is more than
one named branch on the branch that's going to be rebased.

hgext/rebase.py

@@ -175,6 +175,14 b' def rebase(ui, repo, **opts):'
             assert not extrafn, 'cannot use both keepbranches and extrafn'
             def extrafn(ctx, extra):
                 extra['branch'] = ctx.branch()
+            if collapsef:
+                branches = set()
+                for rev in state:
+                    branches.add(repo[rev].branch())
+                    if len(branches) > 1:
+                        raise util.Abort(_('cannot collapse multiple named ' 
+                            'branches'))
+
 
         # Rebase
         if not targetancestors:

tests/test-rebase-collapse.t

@@ -442,3 +442,41 b' Rebase and collapse - B onto F:'
   D
   F
 
+Interactions between collapse and keepbranches
+  $ cd ..
+  $ hg init e
+  $ cd e
+  $ echo 'a' > a
+  $ hg ci -Am 'A'
+  adding a
+
+  $ hg branch '1'
+  marked working directory as branch 1
+  $ echo 'b' > b
+  $ hg ci -Am 'B'
+  adding b
+
+  $ hg branch '2'
+  marked working directory as branch 2
+  $ echo 'c' > c
+  $ hg ci -Am 'C'
+  adding c
+
+  $ hg up -q 0
+  $ echo 'd' > d
+  $ hg ci -Am 'D'
+  adding d
+
+  $ hg tglog
+  @  3: 'D'
+  |
+  | o  2: 'C' 2
+  | |
+  | o  1: 'B' 1
+  |/
+  o  0: 'A'
+  
+  $ hg rebase --keepbranches --collapse -s 1 -d 3
+  abort: cannot collapse multiple named branches
+  [255]
+