Enable searching starlark files with git gs.

Change-Id: Ib4688483cca43b10f88cfc529b03e119b8d2d267
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5119764
Commit-Queue: Gavin Mak <gavinmak@google.com>
Auto-Submit: Garrett Beaty <gbeaty@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

git-gs

@@ -6,4 +6,4 @@ git grep -n -e "$@" -- "*.h" "*.hpp" "*.cpp" "*.c" "*.cc" "*.cpp" "*.inl"\
    "*.grd" "*.grdp" "*.idl" "*.m" "*.mm" "*.py" "*.sh" "*.cfg" "*.tac" "*.go"\
    "*.vcproj" "*.vsprops" "*.make" "*.gyp" "*.gypi" "*.isolate" "*.java"\
    "*.js" "*.ts" "*.html" "*.css" "*.ebuild" "*.pl" "*.pm" "*.yaml" "*.gn"\
-   "*.gni" "*.json" "DEPS" "*/DEPS" "*.mojom" "*.proto" "*.rs"
+   "*.gni" "*.json" "DEPS" "*/DEPS" "*.mojom" "*.proto" "*.rs" "*.star"