REGRESSION: commit logs are not shown sometimes on the new dashboard UI

REGRESSION: commit logs are not shown sometimes on the new dashboard UI
https://bugs.webkit.org/show_bug.cgi?id=138099

Reviewed by Benjamin Poulain.

The bug was caused by _currentItemChanged not passing the previous point in the list of points and also
_showDetails inverting the order of the current and old measurements.

* public/v2/app.js:
(App.PaneController._currentItemChanged): Pass in the previous point to _showDetails when there is one.
(App.PaneController._showDetails): Since points are ordered chronologically, the last point is the
current (latest) measurement and the first point is the oldest measurement.
(App.CommitsViewerComponent.commitsChanged): Don't show a single measurement as a range for clarity.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@175234 268f45cc-cd09-0410-ab3c-d52691b4dbfc