Merge branch 'rs/resolve-ref-optional-result'

Code clean-up.

* rs/resolve-ref-optional-result:
refs: pass NULL to resolve_ref_unsafe() if hash is not needed
refs: pass NULL to refs_resolve_ref_unsafe() if hash is not needed
refs: make sha1 output parameter of refs_resolve_ref_unsafe() optional

diff --cc builtin/receive-pack.c
Simple merge

diff --cc builtin/submodule--helper.c
Simple merge

diff --cc http-backend.c
Simple merge

diff --cc refs.c
Simple merge

diff --cc refs.h
Simple merge

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

diff --cc revision.c
Simple merge