BasePackFetchConnection: Skip object/ref lookups if local repo is empty

BasePackFetchConnection: Skip object/ref lookups if local repo is empty

When cloning repository some of the operations in
`BasePackFetchConnection` can be skipped. We don't need to advertise
packs, compute "wanted time" or wanted refs to send. All of those
operations will try to read objects from an empty repository which
always results in a missing object.

This saves 99.9% of `LooseObjects.open()` calls which dramatically
speeds up object negotiation in V2 protocol.

In testing on JGit (v6.8.0.202311291450-r) repository, which contains
564 refs, the number of calls to `LooseObjects.open()` was reduced from
1187 to 1.

Skipping a call to `markRefsAdvertised()` initially reduced be above
number to 623. Then assuming "0" "want time" an on empty repository
pushed the calls down to 312. Finally, skipping objects reachability on
empty repository set calls down to 1.

The last call is performed from `FetchProcess.asForIsComplete()` which
probably needs to stay in place.

Bug: jgit-5
Change-Id: I2480690726ea54d3b1593380bc8f8d15b4d6afc6