* ChangeLog: Update Git log URL and add a copyright notice.
* Makefile.am (gen-ChangeLog): Use 'gitlog-to-changelog' to generate
the ChangeLog.

Use the 'gitlog-to-changelog' script from Gnulib to generate a
ChangeLog from the Git commit log when creating the distribution
tarball.

* build-aux/gitlog-to-changelog: New file.
* .gitignore: Do not ignore it.
* Makefile.am (gen-ChangeLog): New target.
(dist-hook): Depend on it.
* ChangeLog: New file.