Escape forward slash to avoid undesired path conversion in Git 2.24.1(2).

Bug: none
Change-Id: I8fe6e68ab93025b461722fc6ce43274c091902e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1963407
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>

update_depot_tools

@@ -45,7 +45,7 @@ fi
 
 # We want to update the bundled tools even under MinGW.
 if [ $MINGW = 0 ]; then
-  $COMSPEC /c `cygpath -w "$base_dir/bootstrap/win_tools.bat"`
+  $COMSPEC //c `cygpath -w "$base_dir/bootstrap/win_tools.bat"`
   case $? in
     123)
       # msys environment was upgraded, need to quit.