merge_tools: add merge-tool-conflict-exit-codes option #16864

Job	Run time
build (ubuntu-latest)	5m 2s
Build jj-lib without Git support	48s
build (macos-13)	9m 16s
build (macos-14)	4m 44s
Check protos	36s
Check formatting	17s
build (windows-latest)	10m 22s
build (ubuntu-latest, --all-features)	6m 0s
cargo-deny (advisories)	24s
cargo-deny (bans licenses sources)	25s
Clippy check	2m 32s
Check that MkDocs can build the docs	9s
Check that MkDocs can build the docs with latest Python and uv	21s
	40m 56s

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