meson: wire up benchmarks

Wire up benchmarks in Meson. The setup is mostly the same as how we wire up our tests. The only difference is that benchmarks get wired up via the `benchmark()` option instead of via `test()`, which gives them a bit of special treatment: - Benchmarks never run in parallel. - Benchmarks aren't run by default when tests are executed. - Meson does not inject the `MALLOC_PERTURB` environment variable. Using benchmarks is quite simple: ``` $ meson setup build # Run all benchmarks. $ meson test -C build --benchmark # Run a specific benchmark. $ meson test -C build --benchmark p0000-* ``` Other than that the usual command line arguments accepted when running tests are also accepted when running benchmarks. Note that the benchmarking target is somewhat limited because it will only run benchmarks for the current build. Other use cases, like running benchmarks against multiple different versions of Git, are not currently supported. Users should continue to use "t/perf/run" for those use cases. The script should get extended at one point in time to support Meson, but this is outside of the scope of this series. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>