for BASE=master checkout the one from origin

Previously, the locally-checked-out master got checked out which was
obviously not the intended behaviour.

build.sh

@@ -78,7 +78,12 @@ fi
 progress "Checking out GLUON '$BASE' ..."
 cd $CODE_DIR
 # TODO: check if gluon got modified and bail out if necessary
-git fetch ; git checkout -q $BASE
+git fetch
+if [ "$BASE" = "master" ]; then
+	git checkout -q origin/master
+else
+	git checkout -q $BASE
+fi
 [ "$?" -eq "0" ] || abort "Failed to checkout '$BASE' gluon base version, mimimi." >&2
 GLUON_COMMIT=$(git rev-list --max-count=1 HEAD)