get_snapshot: Support UPSTREAM_GIT_NOUPDATE

And do not manage git when set!

get_snapshot

@@ -37,16 +37,21 @@ if [ -d "${PROJECT}.git/" ]; then
         echo "This is not a GIT repository: $(pwd)" >&2
         exit 1
     fi
+    if [ -z "$UPSTREAM_GIT_NOUPDATE" ]; then
         if [ "$(git config remote.origin.url)" != "${UPSTREAM_GIT_URL}" ]; then
             git remote set-url origin "${UPSTREAM_GIT_URL}"
         fi
         git fetch origin -p
         git checkout -f "${UPSTREAM_GIT_BRANCH}"
         git reset --hard "origin/${UPSTREAM_GIT_BRANCH}"
+    fi
     git clean -fdx
-else
+elif [ -z "$UPSTREAM_GIT_NOUPDATE" ]; then
     git clone -b "${UPSTREAM_GIT_BRANCH}" "${UPSTREAM_GIT_URL}" "${PROJECT}.git/"
     cd "${PROJECT}.git"
+else
+    echo "Missing '${PROJECT}.git' directory!" >&2
+    exit 1
 fi
 
 git_version=$(git describe --tags HEAD)