Merge branch 'ph/fetch-prune-optim'

"git fetch --prune" used to be O(n^2) expensive when there are many
refs, which has been corrected.

* ph/fetch-prune-optim:
  clean up interface for refs_warn_dangling_symrefs
  refs: remove old refs_warn_dangling_symref
  fetch-prune: optimize dangling-ref reporting

diff --cc builtin/fetch.c
Simple merge

diff --cc builtin/remote.c
Simple merge

diff --cc refs.c
Simple merge

diff --cc refs.h
Simple merge