rewriteutil: fix crash when a rewritten message references f{6,64}
Without this, the rewriteutil logic thinks it's found a reference to the wdir
pseudo-revision, and then tries to look it up and rewrite it. Stop it from
doing that.
Amusingly, I had trouble working with this changeset when I didn't
describe the defect above using a regular expression, because it would
trigger the bug in my installed version of hg.
Differential Revision: https://phab.mercurial-scm.org/D11232