Use patches for revisions

[michaelpj]

At the moment revisions are done by providing the whole new cabal file. This is annoying to do, and hard to review. An alternative would be to define revisions as patches.

• The patch should be relative to the last revised cabal file (so you can stack them)
• We probably want to support full cabal files also? For backwards compatibility?
• We could decide whether the file is a cabal file or a patch based on the file extension

For applying patches I think we could either shell out to patch, or maybe there's a library on Hackage that replicates its functionality?

[jasagredo]

In the meantime, one can use this script I put together to review the Consensus update on https://github.com/input-output-hk/cardano-haskell-packages/pull/180

It should be easy to use:

❯ diff-chap-revs diff_all ouroboros-consensus 0.3.0.0
Checked out ouroboros-network at commit d64a5d253b751d465c5c48799dea6b234c2a329a
Diffing revision 1, press something to continue...

Have in mind it just worked for what I needed it to work, it might fail in other esoteric cases!

[yvan-sraka]

I tried to draft an implementation here #51 feel free to comment it :)