We have had this nice utility since 490df753894d (rewriteutil: add a
precheck function to check if revs can be rewritten, 2017-11-24). We
got two callers soon thereafter, in 98f97eb20597 (rewriteutil: use
precheck() in uncommit and amend commands, 2017-11-28). It's about
time we use it in other places.
rewriteutil.precheck() looks unfortunate for i18n (it seems to rely on
an English verb for the action making sense in other languages), but
hopefully not bad enough that we should avoid using it.