rewriteutil: adapt "cannot %s while merging" to work with "change branch of" `rewriteutil.precheck()` creates error messages by inserting a given verb into a sentence. The `hg branch -r` command passes in "change branch of" as the verb. That doesn't work well with "cannot %s while merging" (making it "cannot change branch of while merging"). Let's insert a "changeset" there to make it work better. Building sentences like this seems obviously bad for i18n, but fixing that is out of scope for this series, IMO. Differential Revision: https://phab.mercurial-scm.org/D10530

--- a/mercurial/rewriteutil.py	Thu Feb 11 15:11:10 2021 -0800
+++ b/mercurial/rewriteutil.py	Wed Apr 28 08:48:10 2021 -0700
@@ -44,7 +44,7 @@
         revs = (r.rev() for r in revs)
 
     if len(repo[None].parents()) > 1:
-        raise error.StateError(_(b"cannot %s while merging") % action)
+        raise error.StateError(_(b"cannot %s changesets while merging") % action)
 
     publicrevs = repo.revs(b'%ld and public()', revs)
     if publicrevs: