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

mercurial/rewriteutil.py

@@ -44,7 +44,7 b" def precheck(repo, revs, action=b'rewrit"
         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:

tests/test-commit-amend.t

@@ -406,7 +406,7 b' Refuse to amend during a merge:'
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
   $ hg ci --amend
-  abort: cannot amend while merging
+  abort: cannot amend changesets while merging
   [20]
   $ hg ci -m 'merge'
 

tests/test-uncommit.t

@@ -410,7 +410,7 b' Add and expect uncommit to fail on both '
   [20]
 
   $ hg uncommit --config experimental.uncommitondirtywdir=True
-  abort: cannot uncommit while merging
+  abort: cannot uncommit changesets while merging
   [20]
 
   $ hg status