Add --sync option, to only sync submodules.

crup-runner.sh

 #!/bin/bash
 
+update_toplevel () {
+  # Don't "pull" if checkout is not on a named branch
+  if test "$2" = "pull" && ( ! git symbolic-ref HEAD >/dev/null 2>/dev/null ); then
+    first_args="$1 fetch"
+  else
+    first_args="$1 $2"
+  fi
+  shift 2
+  echo "[$solution] $first_args $@" 1>&2
+  $first_args $@ | sed "s/^/[$solution] /g" 1>&2
+  status=$?
+  if [ "$status" -ne 0 ]; then
+    exit $status
+  fi
+}
+
 set_target_os () {
   # Get the os we're building for.  On first run, this will be unset.
   target_os=$(git config target.os 2>/dev/null)
   if [ -z "$target_os" ]; then
     case $(uname -s) in
       Linux) target_os=unix ;;
       Darwin) target_os=mac ;;
       CYGWIN*|MINGW*) target_os=win ;;
       *)
         echo "[$solution] *** No target.os set in .git/config, and I can't" 1>&2
@@ skipping 50 matching lines @@
 solution="${1%%/*}"
 cd "$solution"
 
 if [ "$solution" = "$1" ]; then
   # Skip git checkouts not managed by crup.
   if ! grep -q -s "The Chromium Authors" ".git/description"; then
     echo "Skipping unmanaged git directory $1" 1>&2
     exit 0
   fi
 
-  # Don't "pull" if checkout is not on a named branch
   shift
-  if test "$2" = "pull" && ( ! git symbolic-ref HEAD >/dev/null 2>/dev/null ); then
-    first_args="$1 fetch"
-  else
-    first_args="$1 $2"
-  fi
-  shift 2
-  $first_args $@ | sed "s/^/[$solution] /g" 1>&2
-  status=$?
-  if [ "$status" -ne 0 ]; then
-    exit $status
+  if test $# -ne 0; then
+    update_toplevel "$@"
   fi
 
   set_target_os
 
   "$GIT_EXE" ls-files -s | grep ^160000 | awk '{print $4}' |
   while read submod; do
     process_submodule "$submod"
   done
   status=$?
 else
   submodule="${1#*/}"
   echo "[$solution] updating $submodule"
   "$GIT_EXE" submodule update --quiet "$submodule" |
   ( grep -v '^Skipping submodule' || true ) | sed "s|^|[$1] |g"
   status=$?
   if [ "$status" -ne "0" ]; then
     echo "[$solution] FAILED to update $submodule"
   fi
 fi
 exit $status