fix(git_changelog_generate): pass tag if it exists to gitReadLog
fix(git_changelog_generate): pass tag if it exists to gitReadLog

Previously if a tag was found the script would try to find commits
between undefined..HEAD. By passing the tag, it now finds tags between
tag..HEAD.

Closes #5.

JohnnyEstilles/git-changelog,fgandellini/git-changelog,rafinskipg/git-changelog,xcambar/git-changelog,richardthombs/git-changelog