refs/files-backend: optimize reading of symbolic refs

When reading references via `files_read_raw_ref()` we always consult both the loose reference, and if that wasn't found, we also consult the packed-refs file. While this makes sense to read a generic reference, it is wasteful in the case where we only care about symbolic references: the packed-refs backend does not support them, and thus it cannot ever return one for us. Special-case reading of symbolic references for the files backend such that we always skip asking the packed-refs backend. We use `refs_read_symbolic_ref()` extensively to determine whether we need to skip updating local symbolic references during a fetch, which is why the change results in a significant speedup when doing fetches in repositories with huge numbers of references. The following benchmark executes a mirror-fetch in a repository with about 2 million references via `git fetch --prune --no-write-fetch-head +refs/*:refs/*`: Benchmark 1: HEAD~ Time (mean ± σ): 68.372 s ± 2.344 s [User: 65.629 s, System: 8.786 s] Range (min … max): 65.745 s … 70.246 s 3 runs Benchmark 2: HEAD Time (mean ± σ): 60.259 s ± 0.343 s [User: 61.019 s, System: 7.245 s] Range (min … max): 60.003 s … 60.649 s 3 runs Summary 'HEAD' ran 1.13 ± 0.04 times faster than 'HEAD~' Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>