[bug-diffutils] keep up with gnulib and tight-scope

[Jim Meyering]

Factoring out README-release, now that it's in gnulib,
using gnulib's new tight-scope rule and removing our
use of its now-removed exit module:

[PATCH 1/4] maint: don't use now-removed gnulib "exit" module
[PATCH 2/4] maint: prepare for the tight-scope check
[PATCH 3/4] build: update gnulib submodule to latest
[PATCH 4/4] maint: use gnulib's new readme-release module