Merge branch 'ps/fetch-mirror-optim'

Various optimization for "git fetch".

* ps/fetch-mirror-optim:
  refs/files-backend: optimize reading of symbolic refs
  remote: read symbolic refs via `refs_read_symbolic_ref()`
  refs: add ability for backends to special-case reading of symbolic refs
  fetch: avoid lookup of commits when not appending to FETCH_HEAD
  upload-pack: look up "want" lines via commit-graph

diff --cc builtin/fetch.c
Simple merge

diff --cc builtin/remote.c
Simple merge

diff --cc refs.c
Simple merge