Merge branch 'it/refs-name-conflict'

Expose "name conflict" error when a ref creation fails due to D/F
conflict in the ref namespace, to improve an error message given by
"git fetch".

* it/refs-name-conflict:
  refs: return conflict error when checking packed refs

diff --cc refs/files-backend.c
Simple merge

diff --cc t/t5510-fetch.sh
Simple merge