R1:6d631577f23c

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 `.