[GHA] Only format Swift files that are in Git index #156

Job	Run time
Benchmarks / 5.8	2m 39s
Benchmarks / 5.9	3m 53s
Benchmarks / 5.10	3m 36s
Benchmarks / nightly-6.0	4m 18s
Benchmarks / nightly-main	4m 58s
Cxx interop / 5.8	1m 3s
Cxx interop / 5.9	1m 8s
Cxx interop / 5.10	1m 8s
Cxx interop / nightly-6.0	1m 47s
Cxx interop / nightly-main	1m 42s
Integration tests / 5.8	8m 20s
Integration tests / 5.9	8m 20s
Integration tests / 5.10	8m 20s
Integration tests / nightly-6.0	8m 19s
Integration tests / nightly-main	8m 20s
Soundness / API breakage check	2m 32s
Soundness / Documentation check	1m 59s
Soundness / Unacceptable language check	3s
Soundness / License headers check	10s
Soundness / Broken symlinks check	5s
Soundness / Format check	32s
Unit tests / 5.8	3m 11s
Unit tests / 5.9	3m 23s
Unit tests / 5.10	3m 13s
Unit tests / nightly-6.0	4m 10s
Unit tests / nightly-main	3m 59s
	1h 31m 8s

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