mklog: handle Signed-off-by, minor cleanup

mklog: handle Signed-off-by, minor cleanup

Consider Signed-off-by lines as part of the ending of the initial
commit to avoid having these in the middle of the log when the
changelog part is injected after.

This is particularly usefull with:

 $ git gcc-commit-mklog --amend -s

that can be used to create the changelog and add the Signed-off-by line.

Also applies most of the shellcheck suggestions on the
prepare-commit-msg hook.

contrib/ChangeLog:

	* mklog.py: Leave SOB lines after changelog.
	* prepare-commit-msg: Apply most shellcheck suggestions.

Signed-off-by: Marc Poulhiès <dkm@kataplop.net>