format/packfile: improve binary delta algorithm

[ajnavarro]

Implement Xdelta algorithm from this paper (http://pop.xmailserver.net/xdfs.pdf) using adler32 checksum for fingerprinting and 16 bytes chunks, like in git.

Fixes #390

[ajnavarro]

Using the same source and target to create the deltas:

• old version: 379373078 ns/op
• new version: 613210 ns/op

[codecov]

Codecov Report

Merging #400 into master will decrease coverage by 0.62%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #400      +/-   ##
==========================================
- Coverage   77.71%   77.09%   -0.63%     
==========================================
  Files         125      124       -1     
  Lines        9106     9001     -105     
==========================================
- Hits         7077     6939     -138     
- Misses       1247     1303      +56     
+ Partials      782      759      -23

Impacted Files	Coverage Δ
plumbing/format/packfile/diff_delta.go	90.74% <100%> (+5.37%)	⬆️
plumbing/transport/ssh/common.go	0% <0%> (-50.91%)	⬇️
plumbing/transport/ssh/auth_method.go	32.4% <0%> (-24.08%)	⬇️
plumbing/format/packfile/patch_delta.go	70.7% <0%> (ø)	⬆️

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[alcortesm]

[alcortesm]

Add a reference to the paper in a comment, so next person to review, modify or use your code knows how to understand it and also to give credit to the original authors.

[ajnavarro]

@alcortesm I added the name of the author and the paper title, because the link can disappear.

[alcortesm]

Excellent call.

I would have added that to the code itself, close to where people will need it, instead of on the commit message.