fetch: avoid second connectivity check if we already have all objects

When fetching refs, we are doing two connectivity checks: - The first one is done such that we can skip fetching refs in the case where we already have all objects referenced by the updated set of refs. - The second one verifies that we have all objects after we have fetched objects. We always execute both connectivity checks, but this is wasteful in case the first connectivity check already notices that we have all objects locally available. Skip the second connectivity check in case we already had all objects available. This gives us a nice speedup when doing a mirror-fetch in a repository with about 2.3M refs where the fetching repo already has all objects: Benchmark #1: HEAD~: git-fetch Time (mean ± σ): 30.025 s ± 0.081 s [User: 27.070 s, System: 4.933 s] Range (min … max): 29.900 s … 30.111 s 5 runs Benchmark #2: HEAD: git-fetch Time (mean ± σ): 25.574 s ± 0.177 s [User: 22.855 s, System: 4.683 s] Range (min … max): 25.399 s … 25.765 s 5 runs Summary 'HEAD: git-fetch' ran 1.17 ± 0.01 times faster than 'HEAD~: git-fetch' Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>