Version control: Improve detection.

Merge managesDirectory() and findTopLevelForDirectory()
into one giving managesDirectory() an optional topLevel
parameter. This removes the need to go up the directory
hierarchy twice when checking for Merurial or git and also
saves some checks for CVS/Subversion.
VCSManager: Check cache in reverse order starting out with
the full path first to improve handling of nested repositories.
Rubber-stamped-by: con
Acked-by: dt