Merge lp://staging/~barry/ubuntu-system-image/lp1444347 into lp://staging/ubuntu-system-image/server

Addresses LP: #1444347 by ensuring that any hardlinks which at first appear not to change between the source and target, but which their underlying link target *does* change, gets properly modified in the diff. This handles the following cases (among others):

source:
a (regular file)
b -> a (via hardlink)

target
a (regular file, modified)
b -> a (via hardlink)

If the diff doesn't not also include b then after application of the diff, b will point to a's *old* inode, but not the "new" a. This branch ensures that in this case, b also gets modified by the diff so that it will point to a's new inode.

Coincidentally, this also fixes a related previously broken case:

source:
a (regular file)
b -> a (via hardlink)

target:
b (regular file)
a -> b (via hardlink)

In this case, both a and b need to be modified in the diff.

This branch includes a few other drive-by improvements.

* Rather than use hard to read nested indexes to get at the bits of information, we use a namedtuple for the internal data structures, which allow us to use easy to read attribute access to ensure we're getting at the bits and pieces we intend to get to.

* Along those lines, this actually fixes some broken indexing in the big "switched hardlinks" conditional. E.g. devminor wasn't being checked previously, nor was gid. Those are now being checked.

* Lots of comments :)

* A few minor code style improvements.

One more important note as you're reviewing this branch. There is a change in an existing test, but I believe it is legitimate. In the original test, the hardlink c/h -> d did not get modified, but this is exactly the case we're trying to fix above. In the test, d does change but the hardlink to d (i.e. c/h) wasn't being changed. That means that after the delta, c/h would point to the old d inode, and we want it to point to the new d.