Add testdata/make/UpdateLeavesTarget

... tests for Leaves targets.
Processor: Print number of targets to update only when > 0
Processor::_CollectMakableTargets(): Fix comment
Processor: Fix handling of Leaves targets

* _PrepareTargetRecursively(): When the dependency has to be made and
  the targets is a Leaves target, only inherit the fate when the
  dependency is a leaf.
* _CollectMakableTargets(): Don't stop recursing a tree branch when
  hitting a target that doesn't have to be made. Dependencies still
  might have to be made.