sanity: Increase minimum git version to 1.8.3.1

sanity: Increase minimum git version to 1.8.3.1

The kernel tools assume git > 1.7.9.5, I'm unsure of the exact version but
the oldest in our infrastructure is 1.8.3.1. The git fetcher also currently
has nasty workarounds for git < 1.7.9.2.

Moving to 1.8.3.1 as our minimum version seems sane at this point as the oldest
we're testing/supporting.

[YOCTO #6162]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>