bookmarks: explicitly track identical bookmarks
bookmarks.compare() previously lumped identical bookmarks in the
"invalid" bucket. This patch adds a "same" bucket.
An 8-tuple for holding this state is pretty gnarly. The return value
should probably be converted into a class to increase readability. But
that is beyond the scope of a patch intended to be a late arrival to
stable.
$ hg init
$ echo a > a
$ hg ci -Am t
adding a
$ hg mv a b
$ hg ci -Am t1
$ hg debugrename b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3
$ hg mv b a
$ hg ci -Am t2
$ hg debugrename a
a renamed from b:37d9b5d994eab34eda9c16b195ace52c7b129980
$ hg debugrename --rev 1 b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3