irmin: allow to keep tree caches after a commit

[samoht]

Expose the clear flag threaded through to Tree.export.

With this, usingS.set_tree_exn ~clear:fase in https://gitlab.com/tezos/tezos/-/merge_requests/8169 seems to fix the issue.

It's also unclear what the default should be - There's an explicit Tree.clear for those who really want to clear the caches, so maybe by default, we should have clear=false?

[codecov-commenter]

Codecov Report

Merging #2225 (d6644ee) into main (8eeffae) will decrease coverage by 0.01%. The diff coverage is 66.66%.

:exclamation: Current head d6644ee differs from pull request most recent head 9d04fc6. Consider uploading reports for the commit 9d04fc6 to get more accurate results

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@@            Coverage Diff             @@
##             main    #2225      +/-   ##
==========================================
- Coverage   68.09%   68.09%   -0.01%     
==========================================
  Files         135      135              
  Lines       16480    16484       +4     
==========================================
+ Hits        11222    11224       +2     
- Misses       5258     5260       +2     

Impacted Files	Coverage Δ
src/irmin/store.ml	66.23% <65.71%> (+<0.01%)	:arrow_up:
src/irmin/tree.ml	81.07% <100.00%> (-0.21%)	:arrow_down:

... and 2 files with indirect coverage changes

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[samoht]

I've added some documentation and tests. It needs #2227 to be able to compare repository configurations in a finer grain as before (as previously we would always cleanup these fields after an export)