Created 'struct ref_lock' to contain the data necessary to perform
a ref update. This change improves writing a ref as the file names
are generated only once (rather than twice) and supports following
symrefs (up to the maximum depth). Further the ref_lock structure
provides room to extend the update API with ref logging.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org> Signed-off-by: Junio C Hamano <junkio@cox.net>