Martin von Zweigbergk <martinvonz@google.com> [Mon, 11 Mar 2019 10:56:56 -0700] rev 47766
fix: rewrite writeworkingdir() to explicitly not work with merges
`hg fix` errors out early if there is an unfinished merge, so we
should have only one parent here. Making that explicit makes my next
patches simpler.
Differential Revision: https://phab.mercurial-scm.org/D11209