[MERGE] Implement hard-link support for branch and checkout

[Stefan Ring]

Thank you so much for this detailed reply.

I already suspected that hard links would be easier to use with the
packs format.

You are absolutely right about the shared repositories feature, but I
would really like the option to "upgrade" a standalone repo to a
shared one. I just tried to do this manually, and while it's not hard
to do, it's tiresome. I would also like to have something like "hg
clone -U" which doesn't build the tree in the cloned repository. The
upcoming "--hardlinks" flag looks like it might help here but as far
as I can tell, it's not yet available in the release version.

Let me share the background why I struggled so much with this. I have
an almost decade-old directory with various stuff collected (or
created) over the years. Now I'd really like to clean it up and also
replicate it to multiple machines for backup purposes. I imported the
whole directory into a bzr repository which is about 4GB large and
unfortunately still in knits format (I started this in the 0.91 days,
IIRC)---I know, it wouldn't be that hard to upgrade but that's not the
point now. Also, I would not consider this to be the typical use-case
for bzr but fast cloning is a very desirable property nonetheless.

Now the problem is this: most of the time, I work on machine1 OR
machine2 while the other one is not reachable (powered down and in
another physical location). I feel really nervous, however, when I
can't replicate my changes at the end of the day because hard disks
die quickly and at the most uncomfortable moments ;)

I thought a bundle against an older revision, that I know the other
machine already has, would help me out of this dilemma. My problem now
was how to create one without having to copy the multi-gigabyte
repository on disk (I don't even have enough free space for that on my
macbook). I have come up with three possible ways to create said
bundle:

1. Create a bundle against an older revision of the repository
itself. Alas, this doesn't work. (-r can be used to specify the source
revision, not the target revision)

2. Convert the repo to a shared repository in-place and then clone it
with -r to an older revision. This works (with quite a bit of manual
file/directory management) but now that "clone -U" option would be
really useful. Also, I've only tried the conversion part, not the
cloning. I really hope that it won't read all 4GB from disk for this
operation.

3. Make a hard-link copy of the whole repository and manually edit the
branch/last-revision file. This also works but again, some manual
editing is involved.

Apart from this bundle dilemma, bzr is the best backup solution I ever
had ;)