Add option to limit blame to first parent
Add unit test
Document enumerator and rewording
Reset num_parents to 1 only for merge commits

Also, correct test case to account for the boundary flag
Remove "ignore" state from the git.git-authors
Merge pull request #2142 from ethomson/ignore_authors

Remove "ignore" state from the git.git-authors
Merge pull request #2137 from jru/blame-first-parent

Blame first-parent history