merge_tools: allow setting conflict marker style per-tool #16650

Job	Run time
build (ubuntu-latest)	5m 7s
Build jj-lib without Git support	50s
build (macos-13)	8m 30s
Check protos	25s
Check that MkDocs can build the docs	9s
build (macos-14)	4m 57s
build (windows-latest)	10m 37s
Check formatting	17s
build (ubuntu-latest, --all-features)	6m 8s
cargo-deny (advisories)	26s
cargo-deny (bans licenses sources)	26s
Clippy check	2m 37s
Check that MkDocs can build the docs with latest Python and uv	15s
	40m 44s

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