This way, the g++ build for the "make presubmit" target gets a separate build directory, and gets to use ccache.

Also, get rid of the "git diff" test, because uncommitted changes are already checked by "git cl upload" and "git cl land".

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1468823002 .