MonthlyCommits is unsafe

[davisjam]

Example: I cloned the popular libuv library and tried to extract monthly commits. I ended up with 16 commits even though the project has had regular commits since 2011 (expected commits = 6 years x 12 months/year = 72 commits).

Explanation: The date being used by RepoDriller uses the author commit time rather than the committer commit time. Suppose someone forks libuv and adds commits. Years later they submit a PR with the fix. Taking a real example, suppose a commit authored in 2015 was merged in 2017. When MonthlyCommits encounters this commit, it will accept it as "at least X months before" the last commit (which was in 2017). This 2015 commit will then cause all of the remaining commits from 2017 and 2016 to be skipped.

Suggested fix: Add both author and committer time fields to ChangeSet. Use the committer time in MonthlyCommits.

Initial exploration: Changing from getAuthorIdent to getCommitterIdent in GitRepository.convertToDate seems to do the right thing: 16 -> 77 commits found, and the timestamps are well-ordered.

Thanks @ayaankazerouni for pointing out the difference between author and committer idents.

[davisjam]

Almost done. @mauricioaniche I'm extending ChangeSet to include message and author/committer objects modeled on JGit's PersonIdent. This will break anyone who uses ChangeSet. Concerns?

See the branch MoreFilters in my fork if you want a sneak preview. Still fixing a test failure.