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