refs: break out ref conflict checks

refs: break out ref conflict checks

Create new function find_descendant_ref, to hold one of the ref
conflict checks used in verify_refname_available. Multiple backends
will need this function, so move it to the common code.

Also move rename_ref_available to the common code, because alternate
backends might need it and it has no files-backend-specific code.

Signed-off-by: David Turner <redacted>
Signed-off-by: Junio C Hamano <redacted>
Signed-off-by: Michael Haggerty <redacted>
Signed-off-by: Jeff King <redacted>