Have git hint with actual diff when committing

Git will add the actual diff to the message displayed in the editor.
This allows to consult better what was actually done and to make use of
code completion features like that of Vim.

Note that with --amend (`cma` alias) the behavior is that the final
diff is shown, i.e. not just the amendment.

Also it's intentionally not used by default message aliases as these
are intended for express and/or ad-hoc commits (so far).