Girocco/Config.pm: mention Git's reference-transaction hook

Girocco/Config.pm: mention Git's reference-transaction hook

The reference-transaction hook added in Git v2.28.0 makes most of
what the $autogchack does unnecessary.  However, based on an
unfortunate design decision in the implemenation of the
reference-transaction hook, Girocco still must keep track of the
old value of any updated refs to generate the correct change notices.

It does this anyway for all mirrors already, not a big deal.

For now, just mention that the reference-transaction hook has been
available since Git v2.28.0; eventually, changes will be made to
take advantage of the new hook to obsolete the $autogchack setting
when running with a Git >= 2.28.0.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>