Merge branch 'jk/symbolic-ref'

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