maint: autogenerate ChangeLog

maint: autogenerate ChangeLog

Following the practice set by various other GNU projects, we start
to automatically generate the ChangeLog file from the git commit
messages.  This will avoid duplication (as the ChangeLog entries
were always inserted both in the git commit message and in the
version-controlled ChangeLog file), and potential problems with
spurious merge conflicts (which, although greatly mitigated by
Bruno Haible's `git-merge-changelog' helper program, have never
been completely solved).

* ChangeLog: Moved ...
* ChangeLog.11:... to this.
* Makefile.am (EXTRA_DIST): Add it.
(gitlog_to_changelog_command, gitlog_to_changelog_options): New
variables.
(ChangeLog, am--changelog-regen-hook): New targets.
* .gitignore: Add ChangeLog.