add caching support to git-daemon

by Nick Edelen for Git Development Community

every time git-daemon handles a fetch, it must (re-)enumerate and (re-)pack all the required objects. this is often very inefficient. I propose maintaining an updated list of commits/objects reachable from HEAD. this would eliminate our need for extensive tree traversals. further, 'partial packfiles' could be used to cache slices of the object list. due to the nature of packfiles, these could more-or-less be coalesced to form the majority of the pack.