templates: fix revset('parents()') % ... in amend message template

I don't understand why, but putting revset('parents()') % {desc} in
the commit message template for amend resulted in a crash because
memctx.hex() did hex(self.node()) and its node was None. This
patch fixes that.

Martin von Zweigbergk <martinvonz@google.com>

Differential Revision: https://phab.mercurial-scm.org/D10707