R1:785f3c98da3a

Extract raw commit messages from Git more faithfully across Git versions

Summary:
Fixes T5028. Older versions of Git (apparently, from before 2010) did not provide a way to extract the raw body of a commit message from "git log", so we approximate it with "subject" and "wrapped body".

In newer versions of Git, the raw body can be extracted exactly.

Adjust how we extract messages based on the version of Git, and try to be more faithful to edge cases: particularly, be more careful to extract the correct…