mailmap: add support for `.mailmap` files #12021

Job	Run time
Build jj-lib without Git support	47s
cargo-deny (advisories)	25s
build (ubuntu-latest)	4m 18s
cargo-deny (bans licenses sources)	27s
build (macos-14)	3m 17s
Check protos	37s
build (windows-latest)	9m 1s
Clippy check	2m 23s
build (ubuntu-latest, --all-features)	5m 9s
Check that MkDocs can build the docs	20s
Check that MkDocs can build the docs with Poetry 1.8	39s
Check formatting	17s
	27m 40s

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