merge_tools: add merge-conflict-exit-codes option #16865

Job	Run time
build (ubuntu-latest)	4m 55s
Build jj-lib without Git support	51s
build (macos-13)	12m 3s
Check protos	31s
build (macos-14)	3m 39s
build (windows-latest)	10m 33s
cargo-deny (advisories)	23s
Clippy check	2m 34s
build (ubuntu-latest, --all-features)	6m 8s
cargo-deny (bans licenses sources)	22s
Check that MkDocs can build the docs	12s
Check that MkDocs can build the docs with latest Python and uv	13s
Check formatting	18s
	42m 42s

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