cmdutil: add a missing byte prefix to string introduce in 976b26bdd0d8

The change is missing a the `b'foo'` prefix to make it a bytestring. This lead
to a traceback in some third party extension. It is unclear to me why the
Mercurial test pass without it.

Differential Revision: https://phab.mercurial-scm.org/D9974

mercurial/cmdutil.py

@@ -3145,7 +3145,7 b' def commitstatus(repo, node, branch, bhe'
         # avoid reporting something like "committed new head" when
         # recommitting old changesets, and issue a helpful warning
         # for most instances
-        repo.ui.warn(_("warning: commit already existed in the repository!\n"))
+        repo.ui.warn(_(b"warning: commit already existed in the repository!\n"))
     elif (
         not opts.get(b'amend')
         and bheads