irmin-git: fix decoding contents in the middle of a buffer

[metanivek]

This code previously assumed that it could read to the end of the buffer, but this is not always the case. This issue was discovered while testing the new batch API in irmin-client.

This PR updates irmin-git to use ocaml-git 3.14.0 or newer. This release of ocaml-git fixes the discovered bug and also exposes a function (length_with_header) that can be used to know the length of buffer read. I also updated the client/server example to use an in-memory git store, which can be used to manually verify this fix.

[codecov-commenter]

Codecov Report

Merging #2277 (08c4142) into main (2e7a6a2) will increase coverage by 0.01%. The diff coverage is 0.00%.

:exclamation: Current head 08c4142 differs from pull request most recent head 39d14fe. Consider uploading reports for the commit 39d14fe to get more accurate results

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@@            Coverage Diff             @@
##             main    #2277      +/-   ##
==========================================
+ Coverage   68.12%   68.14%   +0.01%     
==========================================
  Files         133      133              
  Lines       15982    15983       +1     
==========================================
+ Hits        10888    10891       +3     
+ Misses       5094     5092       -2     

Files	Coverage Δ
src/irmin-git/contents.ml	45.00% <0.00%> (-2.37%)	:arrow_down:

... and 3 files with indirect coverage changes

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[art-w]

Thanks, it looks good! (~I'm guessing the CI doesn't yet see the new release of ocaml-git~ ... ~Hm but actually the commit used for the opam-repository is recent enough, so there might be a conflict in our dependencies~ Nevermind, the dependency analysis went through but now it seems stuck elsewhere)