rebase: add option to not commit after a collapsing

This option is useful when other extensions (e.g., pbranch) want to
use rebase --collapse and append other things before committing.
This is not intended to be used from command line.

hgext/rebase.py

@@ -87,6 +87,9 b' def rebase(ui, repo, **opts):'
         keepf = opts.get('keep', False)
         keepbranchesf = opts.get('keepbranches', False)
         detachf = opts.get('detach', False)
+        # keepopen is not meant for use on the command line, but by
+        # other extensions
+        keepopen = opts.get('keepopen', False)
 
         if contf or abortf:
             if contf and abortf:
@@ -181,7 +184,7 b' def rebase(ui, repo, **opts):'
 
         ui.note(_('rebase merging completed\n'))
 
-        if collapsef:
+        if collapsef and not keepopen:
             p1, p2 = defineparents(repo, min(state), target,
                                                         state, targetancestors)
             commitmsg = 'Collapsed revision'