Handle initial commit

[dtaniwaki]

To support full commits, I used _ as a special character to indicate before the first commit because there's no commit hash for it. When _ is specified for the ComittishFrom variable, git-ghost creates a commits patch by just specifying the ComittishFrom in git log instead of specifying a range. This patch can be applied by git am to a git repo without any commits.

I also refactored the code to implement the special handling easier.

Fixes #5