Fix reading of ref names containing characters that sort before /

Fix reading of ref names containing characters that sort before /

A set of ref names like ('a/b' and 'a+b') would cause the RefDirectory
to think that the set of refs have changed because it traversed the
'a' directory in the subtree before looking at 'a+b', but it then
compared with the know refs which are sorted with 'a+b' first.

Fix this by traversing the refs tree in another order. Treat a directory
as if they ends with a '/' before deciding on the order to traverse
the refs tree.

Bug: 348834
Change-Id: I23377f8df00c7252bf27dbcfba5da193c5403917
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>