Print updating depot_tools in stderr

stdout print statement may interfere with automated tooling. Instead,
this prints the warning to stderr. See: https://crrev.com/c/3556588

R=gavinmak@google.com
Change-Id: I3fb72b7ab4afe42cb83ea5e506292f3e6e5ea8d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3561445
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

update_depot_tools

@@ -125,7 +125,7 @@ function update_git_repo {
 
 # Update git checkouts.
 if [ "X$DEPOT_TOOLS_UPDATE" != "X0" ]; then
-  echo "Updating depot_tools..."
+  echo "Updating depot_tools..." 1>&2
   if [ -e "$base_dir/.git" ]; then
     cd "$base_dir"
     update_git_repo