The history entry symbol (e.g., + or -) column is now only shown in

the DiffCommitIndexView when there is a commit history diff to show.

This results in less whitespace on the left hand side of the table and

the diff complexity icons will align with those in the

DiffFileIndexView.

Verified the extra column is not shown when there is no commit diffing

being done, or when the two histories are identical.