Merge branch 'mh/init-delete-refs-api'

Clean up refs API and make "git clone" less intimate with the
implementation detail.

* mh/init-delete-refs-api:
delete_ref(): use the usual convention for old_sha1
cmd_update_ref(): make logic more straightforward
update_ref(): don't read old reference value before delete
check_branch_commit(): make first parameter const
refs.h: add some parameter names to function declarations
refs: move the remaining ref module declarations to refs.h
initial_ref_transaction_commit(): check for ref D/F conflicts
initial_ref_transaction_commit(): check for duplicate refs
refs: remove some functions from the module's public interface
initial_ref_transaction_commit(): function for initial ref creation
repack_without_refs(): make function private
prune_refs(): use delete_refs()
prune_remote(): use delete_refs()
delete_refs(): bail early if the packed-refs file cannot be rewritten
delete_refs(): make error message more generic
delete_refs(): new function for the refs API
delete_ref(): handle special case more explicitly
remove_branches(): remove temporary
delete_ref(): move declaration to refs.h

diff --cc builtin/blame.c
Simple merge

diff --cc builtin/clone.c
Simple merge

diff --cc builtin/log.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc refs.c
Simple merge