logcmdutil: call _exthook() in changesettemplater

Class changesetprinter has an _exthook() method that is called in
_show() before the patch is displayed. Call the method as well in
changesettemplater._show().

Differential Revision: https://phab.mercurial-scm.org/D7505

mercurial/logcmdutil.py

@@ -598,6 +598,7 b' class changesettemplater(changesetprinte'
         # write changeset metadata, then patch if requested
         key = self._parts[self._tref]
         self.ui.write(self.t.render(key, props))
+        self._exthook(ctx)
         self._showpatch(ctx, graphwidth)
 
         if self._parts[b'footer']: