To avoid merge conflicts, we want to avoid modifying the file on
multiple branches in parallel. This patch is therefore meant to be
applied to the stable branch and then quickly be merged to default (at
least before edits are made to relnotes/next there).

Another option would have been to copy the file on the stable branch
and to clear it on the default branch. However, that still results in
conflicts if the copy is edited on the stable branch (Mercurial would
try to apply the changes from the default branch to it).

We could also delete the file in one commit and recreate it in another
commit. However, Mercurial is quite inconsistent in what it considers
a break in history (see test-copies-unrelated.t), so I'd like to avoid
that.

Differential Revision: https://phab.mercurial-scm.org/D6705