split magit-commit-add-log in two

Add new function `magit-commit-add-log-insert' from code
previously in `magit-commit-add-log'.

Add new variable `magit-commit-add-log-insert-function' allowing
users to use something else than this new function to insert the
entry, without having to duplicate the parts that remain in the
command `magit-commit-add-log'.