maint: Stop generating ChangeLog from git

maint: Stop generating ChangeLog from git

Our ChangeLog is generated by basically redirecting the output
of 'git log' into it so, as can be expected, it has only gotten
bigger as development has progressed. As of today, its size has
reached pretty much comical levels:

  $ du -sk ChangeLog
  11328 ChangeLog

All of that for information *literally nobody* cares about: end
users and distro maintainers have proper release notes lovingly
compiled for them, while developers peruse the history either by
calling 'git log' directly or through their favorite $EDITOR's
git integration.

Replacing the generated ChangeLog with a short message pointing
interested parties to the git repository does not only reduce
the size of the unpacked sources from 259904 KiB to 248576 KiB
(~4% saving): from a quick test on my laptop, doing so reduces
the size of the *compressed* release archive from 15140 KiB to
12364 KiB (~18% saving) and also takes the time needed to run
'make distcheck' down from 4:44 to 4:21 (~8% saving).

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>