Merge branch 'jk/symbolic-ref'

The low-level code that is used to create symbolic references has been updated to share more code with the code that deals with normal references. * jk/symbolic-ref: lock_ref_sha1_basic: handle REF_NODEREF with invalid refs lock_ref_sha1_basic: always fill old_oid while holding lock checkout,clone: check return value of create_symref create_symref: write reflog while holding lock create_symref: use existing ref-lock code create_symref: modernize variable names