SideBySide2: Track SkipBar instances by the starting line

Use a SortedMap to track the SkipBars, indexed by first line number
hidden by the bar. This simple refactoring allows for a future
change to find if a line is covered by a skip bar.

Change-Id: Iee71856e0c24140a10c0ee4938abfcef6490d6e9
2 files changed