packfile: improve performance of delta generation

[erizocosmico]

Caveats

I kept the old API for deltas untouched and added some private methods that reuse indexes, so the methods can be used outside and the index is automatically created.

I ported a modified version of JGit's DeltaIndex and DeltaIndexScanner that is faster (not much more, though) and, somehow, make the encode process faster. A process that consistently took ~2m30s before now takes ~1m with this. This might be caused by a bug in the index, of course, but everything seems to be ok according to the tests and some manual check I did with the aforementioned process.

[codecov]

Codecov Report

Merging #582 into master will decrease coverage by 0.29%.
The diff coverage is 96.49%.

@@            Coverage Diff            @@
##           master     #582     +/-   ##
=========================================
- Coverage   78.04%   77.75%   -0.3%     
=========================================
  Files         129      130      +1     
  Lines        9963    10097    +134     
=========================================
+ Hits         7776     7851     +75     
- Misses       1341     1414     +73     
+ Partials      846      832     -14

Impacted Files	Coverage Δ
plumbing/format/packfile/diff_delta.go	90.47% <100%> (-1.26%)	⬇️
plumbing/format/packfile/delta_index.go	100% <100%> (ø)
plumbing/format/packfile/delta_selector.go	78.94% <79.31%> (-0.59%)	⬇️
plumbing/transport/ssh/common.go	20.54% <0%> (-45.21%)	⬇️
plumbing/transport/ssh/auth_method.go	31.57% <0%> (-22.81%)	⬇️
plumbing/format/packfile/patch_delta.go	76.61% <0%> (+4.83%)	⬆️

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[erizocosmico]

This is ready for review now

[ajnavarro]

👏 👏 👏 👏