The "perf-lib.sh" script is sourced by all of our benchmarking suites to make available common infrastructure. The script assumes that build and source directory are the same, which works for our Makefile. But the assumption breaks with both CMake and Meson, where the build directory can be located in an arbitrary place. Adapt the script so that it works with out-of-tree builds. Most importantly, this requires us to figure out the location of the build directory: - When running benchmarks via our Makefile the build directory is the same as the source directory. We already know to derive the test directory ("t/") via `$(pwd)/..`, which works because we chdir into "t/perf" before executing benchmarks. We can thus derive the build directory by appending another "/.." to that path. - When running benchmarks via Meson the build directory is located at an arbitrary location. The build system thus has to make the path known by exporting the `GIT_BUILD_DIR` environment variable. This change prepares us for wiring up benchmarks in Meson. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>