add flags to control whether duplicates in history should be preserved

[phercek]

Add flags historyIgnoreDuplicates and historyIgnoreAllDuplicates.

historyIgnoreDuplicates Do not enter command lines into the history list if they are duplicates of the previous event.

historyIgnoreAllDuplicates If a new command line being added to the history list duplicates an older one, the older command is removed from the list (even if it is not the previous event).

This is just to keep the usefull stuff in history so that users do not need to have it too long to make sure interesting items are not evicted by repeated use of few commands. Even the next/previous history item retrieval works better with these.

[judah]

Done now in HEAD; documented at UserPrefs.

Thanks for the suggestion; at first this didn't seem like a big deal to me, but after using it for a half-hour I really like it.