Adjust scale on Visual History scrubber to match zoom

[justinrobots]

Description

When zoomed in to Visual History, scrubbing can get very sensitive because the scrubber is scaled to the total time frame (e.g. 3 months) even if the view is zoomed to show a week. In that case, small movements on the scrubber translate to large movements in the view, which make it difficult to carefully select commits. It would be nice if the scrubber’s scale adjusted to the zoom so I could more accurately move through diffs.