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