[PATCH] Disable pointless menu entry

What about disabling the menu entry for "purge history"? It actually doesn't
purge the *history*, but the *revisions* and is a duplicate of the menu entry
"purge revisions" which does exactly the same (and has the right label).
There doesn't seem to be a function that purges all history, and it is
confusing for the user to have a menu entry that doesn't do what it says but
something completely different.

Ingo