rebase: improve error message for more than one external parent

hgext/rebase.py

@@ -406,8 +406,10 b' def externalparent(repo, state, targetan'
         return nullrev
     if len(parents) == 1:
         return parents.pop()
-    raise util.Abort(_('unable to collapse, there is more '
-                       'than one external parent'))
+    raise util.Abort(_('unable to collapse on top of %s, there is more '
+                       'than one external parent: %s') %
+                     (max(targetancestors),
+                      ', '.join(str(p) for p in sorted(parents))))
 
 def concludenode(repo, rev, p1, p2, commitmsg=None, editor=None, extrafn=None):
     'Commit the changes and store useful information in extra'

tests/test-rebase-collapse.t

@@ -226,7 +226,7 b' Rebase and collapse - more than one exte'
   $ cd b1
 
   $ hg rebase -s 2 --collapse
-  abort: unable to collapse, there is more than one external parent
+  abort: unable to collapse on top of 7, there is more than one external parent: 1, 5
   [255]
 
 Rebase and collapse - E onto H: