`libgit2` requires `libssh2`, which in turn requires `openssl-sys`. OpenSSL is
notoriously hard to vendor for a number of reasons including its build system.
In contrast, while BoringSSL does not make compatibility guarantees, it is easy
to vendor and is designed to be used with Bazel.
The goal is that we can substitute BoringSSL for OpenSSL in `openssl-sys` as the
underlying library, and `libssh2` will still work.
Signed-off-by: Austin Seipp <aseipp@pobox.com>
