maint: improve generation of ChangeLog from git commitsbranch-2.0

Follow the practice set in numerous other GNU projects, where
the ChangeLog (since 2015) is generated from git commit messages.
This avoids duplication or subtle differences between the two,
as well as making it easier to merge patches across branches
(as good as Bruno Haible's 'git-merge-changelog' helper program
is, it still doesn't handle cross-branch cherry-picks very well).

* Makefile.am (EXTRA_DIST): Rely on gnulib to ship gitlog-to-changelog.
($(changelog)): Update rule to match branch-1.4 style.
* .gitattributes: Drop stale mention of Changelog.
* HACKING: Reword to match new procedure, and simplify by
referring to an external description of ChangeLog style.

Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit 5456676ea90dfedd9b54c843f15b006d02150d06)
[commit 6bdb0a4b75 already started the conversion, but this updates
the approach to be more like other GNU projects, and branch-1.4]

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