Version Control With Subversion Source

* src/en/book/ch05-repository-admin.xml
(Revision files and shards): New sidebar, gutted from the text
around fsfs-reshard.py, and amended to also include a mention of
the new packed format.
(fsfs-reshard.py): Refer to new sidebar now when talking about
revision files and shards.
(Packing FSFS filesystems): Refer to new sidebar.

Suggested by: Neels J Hofmeyr <neels{_AT_}elego.de>