HOW TO COMMIT CHANGES TO EMACS

(0) Each commit should correspond to a single change (whether spread
    over multiple files or not).  Do not mix different changes in the
    same commit (eg adding a feature in one file, fixing a bug in
    another should be two commits, not one).

(1) Commit all changed files at once with a single log message (the
    default behavior in git).

(2) Make the log message describe the entire changeset. The log
    message should be the same as the Changelog entry, without the
    date line. You can write the Changelog first, then use C-c C-a
    from the *VC-Log* buffer to copy it to the commit log message. See
    the file 'changelog' for information on the Changelog entry
    format.

(3) If committing changes written by someone else, make the ChangeLog
    entry in their name, not yours. git distinguishes between the
    author and the committer; use the --author option on the commit
    command to specify the actual author; the committer defaults to
    you.

PREVIOUS GUIDELINES FOR CVS

For historical interest only, here is the old-style advice for CVS logs:
http://lists.gnu.org/archive/html/emacs-devel/2007-12/msg01208.html