appveyor: Bump shallow clone depth.

appveyor: Bump shallow clone depth.

To prevent build failures when a large patch series is committed, like
happened in https://ci.appveyor.com/project/jrfonseca-fdo/mesa/build/322
due to 10 commits between dac2964f3ebd96d5ac227984ab0cd79c2c3b2a1a and
6f428328d34bed16edb8709e4a117eb710d7893d where submitted before the
build slave started the git clone.

100 commits should be bigger than any patch series seen in practice, and
it takes practically the same time to download as 5 commits.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>