help: report on whether or not Rust is enabled

We're about to introduce support for Rust into the core of Git, where
some (trivial) subsystems are converted to Rust. These subsystems will
also retain a C implementation though as Rust is not yet mandatory.
Consequently, it now becomes possible for a Git version to have bugs
that are specific to whether or not it is built with Rust support
overall.

Expose information about whether or not Git was built with Rust via our
build info. This means that both `git version --build-options`, but also
`git bugreport` will now expose that bit of information. Hopefully, this
should make it easier for us to discover any Rust-specific issues.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>