help: include SHA implementation in version info

help: include SHA implementation in version info

When the `--build-options` flag is used with git-version(1), additional
information about the built version of Git is printed. During build
time, different SHA implementations may be configured, but this
information is not included in the version info.

Add the SHA implementations Git is built with to the version info by
requiring each backend to define a SHA1_BACKEND or SHA256_BACKEND symbol
as appropriate and use the value in the printed build options.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>