search: skip working copy meta data of known version control systems

[ossi]

most of the time there is no point in searching through the repository data of the version control systems.

this patch is somewhat crude - the list should be probably come from a config file. the list isn't complete, either - it covers only the VCS i'm using.
one might consider adding a checkbox (like the "skip hidden") to the actual search dialog, but i think this is pretty much pointless and thus not worth the ui clutter.