verify: use some intermediate variables instead of a multi-liner

This is shorter and easier to read as the indentation remains the same.

Differential Revision: https://phab.mercurial-scm.org/D10816

mercurial/verify.py

@@ -378,12 +378,9 b' class verifier(object):'
                 if dir:
                     self._err(c, WARN_PARENT_DIR_UNKNOWN_REV % short(m), label)
                 else:
-                    self._err(
-                        c,
-                        _(b"changeset refers to unknown revision %s")
-                        % short(m),
-                        label,
-                    )
+                    msg = _(b"changeset refers to unknown revision %s")
+                    msg %= short(m)
+                    self._err(c, msg, label)
 
         if not dir and subdirnodes:
             self.ui.status(_(b"checking directory manifests\n"))