Detect changes in merge commits as the diff of the merge and the first parent
Summary:
Currently, we use "git log" to detect the change list for all commits, but this produces no output for merge commits.
Instead, parse them as changes against the first parent (the merge destination). This produces generally sensible/expected behavior, and is consistent with what GitHub does.
We need to special-case the first commit because it doesn't have parents.
NOTE: This is a parser change so you need to run ./scripts/repository/reparse.php --all <callsign> --change to reparse merge commits in already-imported repositories after updating.
Test Plan: Reparsed a merge commit, a non-merge commit, and the first commit in the Phabricator repository.
Reviewers: btrahan, gschmidt
Reviewed By: btrahan
CC: aran, epriestley
Maniphest Tasks: T961
Differential Revision: https://secure.phabricator.com/D1985