revfile developments

[Martin Pool]

Hi Matt,

Thanks for letting me use your revfile code.

Here are the changes I made:

http://bazaar-ng.org/bzr/bzr.revfile/bzrlib/mdiff.py
http://bazaar-ng.org/bzr/bzr.revfile/bzrlib/revfile.py

At first I tried doing a byte-by-byte diff, but that turns out to be too
slow, as you probably know.  I fixed a bug in the linesplit()
function.  

There are two small optimizations to avoid storing a diff or avoid doing
gzip if they wouldn't win.

I think it's important to be able to have branching within the storage
of a single file, so I added that.

Although I index by SHA-1, I don't make the mistake of Monotone of
assuming that two objects with the same content are the same thing.
There is a higher-level inventory and revision object that just uses
revfile as a content-addressible store.  I need to use something more
than just an integer to identify revisions because it's too hard to keep
simple integers in sync in a distributed system.  A nice side effect is
that we can easily check we're getting out the text we meant to put in.

Each delta has a "base" pointer saying which previous text it's stored
relative to.  The base pointer doesn't have any meaning to the revision
control layer; it's just for delta compression.  This could be
manipulated to do some kind of skip-deltas to avoid ever needing to
store the full text, but I don't do that for now.

-- 
Martin

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