Warn on repo upload if you're not uploading to AOSP master.

Should help save the "please upload against AOSP master" first
conversations with people trying to make changes.

Test: manual
Change-Id: I3d4bfcb0379da12a93e65a3621edda3bbde72a88

PREUPLOAD.cfg

@@ -6,5 +6,6 @@ clang_format = true
 clang_format = --commit ${PREUPLOAD_COMMIT} --style file --extensions c,h,cc,cpp
 
 [Hook Scripts]
+aosp_hook = ${REPO_ROOT}/frameworks/base/tools/aosp/aosp_sha.sh ${PREUPLOAD_COMMIT} "."
 checkstyle_hook = ${REPO_ROOT}/prebuilts/checkstyle/checkstyle.py --sha ${PREUPLOAD_COMMIT}
                   --file_whitelist tools/ updater_sample/