completion: add support for new "amend" command

The command is now shipped with Mercurial, but completion should be
helpful (and accurate) for users of the amend command shipped with the
evolve extension too.

Differential Revision: https://phab.mercurial-scm.org/D1716

contrib/bash_completion

@@ -296,7 +296,7 b' shopt -s extglob'
         merge)
             _hg_labels
         ;;
-        commit|ci|record)
+        commit|ci|record|amend)
             _hg_status "mar"
         ;;
         remove|rm)