rebase: add a "D" short option for detach

Detach is usually what I want when I use --source or (in particular) --rev.
Having a shorter option make it less an hassle to use it.

hgext/rebase.py

@@ -47,7 +47,7 b' command = cmdutil.command(cmdtable)'
      _('read collapse commit message from file'), _('FILE')),
     ('', 'keep', False, _('keep original changesets')),
     ('', 'keepbranches', False, _('keep original branch names')),
-    ('', 'detach', False, _('force detaching of source from its original '
+    ('D', 'detach', False, _('force detaching of source from its original '
                             'branch')),
     ('t', 'tool', '', _('specify merge tool')),
     ('c', 'continue', False, _('continue an interrupted rebase')),