bst track reports strange output for untrackable elements

Removing the track: parameter from an element mostly works as expected, and can sometimes be useful for pinning a given element at a specific version, avoiding any updates at bst track time.

However the output of this is strange:

[--:--:--][????????][track:base/ninja.bst            ] INFO    Found new revision: None

Rather than claiming to have found a new revision; with an INFO message, we should just mention that tracking is skipped in a STATUS message.