meson: wire up development environments

The Meson build system is able to wire up development environments. The intent is to make build artifacts of the project available. This is typically used to export e.g. paths to linkable libraries, which isn't all that interesting in our context given that we don't have an official library interface. But what we can use this mechanism for is to expose the built Git executables as well as the build directory. This allows users to play around with the built Git version in the devenv, and allows them to execute our test scripts directly with the built distribution. Wire up this feature, which can then be used via `meson devenv` in the build directory. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>