Change Ref to track name, not full file location

Change Ref to track name, not full file location

Typically in Git we talk about refs as being relative to some
particular repository, hence their actual location on disk can
vary based on where they are stored.  Core Git also supports a
"packed-refs" file that allows multiple refs to be stored in
one file stream; in such a case the refs are distinct by name
but share the same location on disk.

Storing the name instead of the file location makes the Ref
class more flexible..

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>