clear the finished or in progress computation.
Limit: we may receive in cache the result of a computation
which started before the clear (if this is a problem we could
store a cleaning counter alongside the cache to prevent insertions)
return the closest parent (or self) containing a .git file
try to get the result of the computation of the tree git status.
This may be immediate if a previous computation was finished.
This may wait for the result of a new computation or of a previously
launched one.
In any case: