gsutil.py.bat: Skip updating depot_tools if DEPOT_TOOLS_UPDATE=0.

Bug: 1090603
Change-Id: I05ce5628320b36bf81fe7028d8c84427758395f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2229064Reviewed-by: Arthur Wang <wuwang@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

gsutil.py.bat

@@ -4,6 +4,9 @@
 :: found in the LICENSE file.
 setlocal
 
+:: Shall skip automatic update?
+IF "%DEPOT_TOOLS_UPDATE%" == "0" GOTO :CALL_GSUTIL
+
 :: Synchronize the root directory before deferring control back to gsutil.py.
 call "%~dp0update_depot_tools.bat" %*
 :: Abort the script if we failed to update depot_tools.
@@ -11,6 +14,7 @@ IF %errorlevel% NEQ 0 (
   goto :EOF
 )
 
+:CALL_GSUTIL
 :: Ensure that "depot_tools" is somewhere in PATH so this tool can be used
 :: standalone, but allow other PATH manipulations to take priority.
 set PATH=%PATH%;%~dp0