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/tests/test-uncommit.t	Thu Feb 11 15:11:10 2021 -0800
+++ b/tests/test-uncommit.t	Wed Apr 28 08:48:10 2021 -0700
@@ -410,7 +410,7 @@
   [20]
 
   $ hg uncommit --config experimental.uncommitondirtywdir=True
-  abort: cannot uncommit while merging
+  abort: cannot uncommit changesets while merging
   [20]
 
   $ hg status