Recycle project_root output

_reformat-common.bash

@@ -16,8 +16,16 @@ function get_os() {
 
 # For a given file, return its corresponding project root.
 function project_root() {
+    set +u
+    if [[ ! -z "${PROJECT_ROOT}" ]]; then
+        echo "$PROJECT_ROOT"
+        return 0
+    fi
+    set -u
+
     pushd "$(dirname "$1")" >/dev/null 2>&1 || exit 101
     out=$(git rev-parse --show-toplevel)
+
     ret="$?"
     popd >/dev/null 2>&1 || exit 1