Lessen focus on ChangeLog files, as opposed to change log entries.

This is in preparation for generating the former automatically
from the latter.
* admin/notes/bugtracker, admin/notes/copyright, admin/notes/newfile:
ChangeLog -> change log
* admin/notes/changelogs: Remove, merging old contents to ...
* admin/notes/repo: ... here.
* doc/emacs/maintaining.texi (Change Log): Mention that ChangeLog files may
be copied to or from a version control system.
* doc/emacs/trouble.texi (Sending Patches): Point to the commit messages.
* doc/lispref/intro.texi (Acknowledgments): ChangeLog file -> change log entries.
* doc/lispref/tips.texi (Library Headers): Emacs uses a version control system.
* etc/CONTRIBUTE: Give advice about git commit messages and how
to generate proposed patches containing them.