Replace A and B with First, Second, Selected, Parent + commit message beginning.

Handle added and deleted files. Closes  #1917.
Check if git is still affected by RaceConditionWhenGitStatusIsUpdatingIndex.

As per comment: https://github.com/gitextensions/gitextensions/commit/8b47b8c76ec1ebe9668051f3175bb31bd8289d68#commitcomment-7674932