mailmap: add support for `.mailmap` files #12016

Job	Run time
build (ubuntu-latest)	3m 59s
build (macos-14)	3m 9s
build (windows-latest)	8m 16s
build (ubuntu-latest, --all-features)	5m 9s
Build jj-lib without Git support	44s
cargo-deny (advisories)	25s
Check protos	40s
cargo-deny (bans licenses sources)	25s
Clippy check	2m 22s
Check that MkDocs can build the docs	25s
Check that MkDocs can build the docs with Poetry 1.8	24s
Check formatting	17s
	26m 15s

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