Work around breaking changes to the changeset API in Stash 2.12

In Stash 2.12, the Changeset API was modified such that Changeset.getAuthor()
and Blame.getAuthor() return StashUser instances when a Stash user can be
found with an e-mail address that matches the committer's e-mail address.

Unfortunately, StashUser violates the Liskov Substitution Principal; StashUser.getName()
returns the committer's Stash username (eg, landonf), rather than the actual
Git committer name as was previously returned by Person.getName().

The actual Git committer name is now discarded; as a result, there's no
way to validate the changeset committer's name directly through the
Stash API in Stash 2.12+.

To resolve this, we now use JGit to fetch the actual committer name directly,
which should work on all supported Stash versions.

For more details, see also:
https://answers.atlassian.com/questions/278040/enforce-push-author-may-break-with-latest-version-of-stash-2-12-the-require-matching-display-name-option-does-not-seem-to-work
Merge pull request #6 from landonf/issue-5-commit-author-check

Work around breaking changes to the changeset API in Stash 2.12