git: force submodule update so local changes/files are overwritten

Without this, we sometimes get "Your local changes to the following
files would be overwritten by checkout" when submodules are updates.

On the non-submodule side, we're already using reset instead of
checkout to avoid this.

Signed-off-by: Cody P Schafer <dev@codyps.com>
Merge jmesmon/buildbot:fix-submod (PR #1167)