Avoid =~ since msysgit bash doesn't support it.

This uses grep instead.  Note that just grepping for "--force" matches other
things, e.g. "foo--force--foo", but that's a problem with the original code as
well, I think.

BUG=435729
TEST=Run gclient under msysgit bash.  No error should occur.

Review URL: https://codereview.chromium.org/789353002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@293340 0039d316-1c4b-4281-b951-d872f2087c98

update_depot_tools

@@ -140,7 +140,7 @@ if [ "X$DEPOT_TOOLS_UPDATE" != "X0" -a -e "$base_dir/.svn" ]
 then
   # Update the root directory to stay up-to-date with the latest depot_tools.
   BEFORE_REVISION=$(get_svn_revision)
-  if [[ $* =~ '--force' ]]; then
+  if echo $* | grep "--force" > /dev/null; then
     "$SVN" -q revert -R "$base_dir"
   fi
   "$SVN" -q up "$base_dir"