gh_metric_commits_prepost_midpoint.RdCalculates the difference in commits between first and last half of a projects calendar life
gh_metric_commits_prepost_midpoint(commits)A dataframe of commits
full_name: repo full name
commits: total commits
commits_prepost_midpoint: Using time from first to last commit as the time scale, what is total commits in first half - commit in second half.
Uses columns fullname, date.