Rename confusing commit_message methods

`#commit_message` returns the raw (read straight from the file) message,
while `#user_commit_message` contains the stuff that the user entered
(stripping out commas and `git diff` output from `git commit
 --verbose`).

We almost always want to work with the 'user' version, so I renamed that
to `commit_message`, and left the raw version exposed as
`raw_commit_message`.

Change-Id: Ia1771c00f88da2e59aa54c45bb32114913f9005f
Reviewed-on: https://gerrit.causes.com/23529
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>