Save the buffer in the commit function
Simplify commit handling in Log Edit Mode

Instead of substituting the key definition, just override
`git-commit-commit-function` locally to call out to
`magit-log-edit-commit`.
Release Git Commit Mode 0.12
Fix commit in Magit Log Edit mode

We must really substitute the C-c C-c binding in Magit Log Edit mode to
bring in our style checking.