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