Merge branch 'kn/ref-transaction-symref' into kn/update-ref-symref

* kn/ref-transaction-symref:
  refs: remove `create_symref` and associated dead code
  refs: rename `refs_create_symref()` to `refs_update_symref()`
  refs: use transaction in `refs_create_symref()`
  refs: add support for transactional symref updates
  refs: move `original_update_refname` to 'refs.c'
  refs: support symrefs in 'reference-transaction' hook
  files-backend: extract out `create_symref_lock()`
  refs: accept symref values in `ref_transaction_update()`

diff --cc branch.c
Simple merge

diff --cc builtin/fast-import.c
Simple merge

diff --cc builtin/fetch.c
Simple merge

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

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

diff --cc t/t0610-reftable-basics.sh
Simple merge