bug#19113: Generate a ChangeLog file from commit logs

[Pádraig Brady]

gitlog-to-changelog takes the same options as `git log`
so it should be easy enough to generate logs for the
last n versions, or the last x versions in the last N years etc.
and discard anything older (which can/should be looked up in git if required).

I'm considering removing any of these old logs
from coreutils that are already in git logs:

$ find -name "ChangeLog*" | xargs du -hsc | sort -h
8.0K    ./build-aux/ChangeLog-2007
8.0K    ./po/ChangeLog-2007
16K     ./ChangeLog-2008
64K     ./old/sh-utils/ChangeLog.0
72K     ./doc/ChangeLog-2007
152K    ./ChangeLog-2006
156K    ./ChangeLog-2007
164K    ./old/sh-utils/ChangeLog
172K    ./m4/ChangeLog-2007
220K    ./old/fileutils/ChangeLog-1997
288K    ./old/fileutils/ChangeLog
304K    ./old/textutils/ChangeLog
312K    ./lib/ChangeLog-2007
440K    ./ChangeLog-2005
1.1M    ./ChangeLog
3.4M    total

Pádraig.