Girocco/Config.pm: add jgit_compatible_bitmaps note
Since the JGit v3.5.0 release in late 2014, JGit can deal with
Git's bitmap hash cache. In fact, since Git v2.22.0 (released
2019-06-07), Git enables the hash cache by default since that's
approximately 5 years since JGit learned to deal with it.
Mention this information in the comments for the obscure
$Girocco::Config::jgit_compatible_bitmaps setting to provide more
guidance about when it may actually need to be activated.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>