gpg: make a message translatable

This message has been overlooked by check-code, because it starts with
non-alphabet character ('%').

This is also a part of preparation for making "missing _() in ui
message" detection of check-code more exact.

hgext/gpg.py

@@ -198,7 +198,7 b' def sigcheck(ui, repo, rev):'
         return
 
     # print summary
-    ui.write("%s is signed by:\n" % hgnode.short(rev))
+    ui.write(_("%s is signed by:\n") % hgnode.short(rev))
     for key in keys:
         ui.write(" %s\n" % keystr(ui, key))