Further preparatory work on the refs API before the pluggable
backend series can land.
* mh/split-under-lock: (33 commits)
lock_ref_sha1_basic(): only handle REF_NODEREF mode
commit_ref_update(): remove the flags parameter
lock_ref_for_update(): don't resolve symrefs
lock_ref_for_update(): don't re-read non-symbolic references
refs: resolve symbolic refs first
ref_transaction_update(): check refname_is_safe() at a minimum
unlock_ref(): move definition higher in the file
lock_ref_for_update(): new function
add_update(): initialize the whole ref_update
verify_refname_available(): adjust constness in declaration
refs: don't dereference on rename
refs: allow log-only updates
delete_branches(): use resolve_refdup()
ref_transaction_commit(): correctly report close_ref() failure
ref_transaction_create(): disallow recursive pruning
refs: make error messages more consistent
lock_ref_sha1_basic(): remove unneeded local variable
read_raw_ref(): move docstring to header file
read_raw_ref(): improve docstring
read_raw_ref(): rename symref argument to referent
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