magit-commit-log-buffer: new function

It would be better to define this as `git-commit-get-buffer' in
`git-commit.el' but we cannot do that because that library should
not depend on Magit and we need `magit-get-top-dir'.
magit-diff-while-{amending,committing}: new function/command
Merge branch 'n/diff-while-committing' into next