Add rs files to git gs.

Rust support is being tested out in Chromium (crbug.com/1292073). This
CL adds support for searching rust (.rs) files when using git gs.

Bug: 1292064
Change-Id: I5e2c5aa16e4dfa5f51a632afda7bdb5ab263b6ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3425617
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Auto-Submit: Dan Sinclair <dsinclair@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Dirk Pranke <dpranke@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" "*.html" "*.css" "*.ebuild" "*.pl" "*.pm" "*.yaml" "*.gn" "*.gni"\
-   "*.json" "DEPS" "*/DEPS" "*.mojom" "*.proto"
+   "*.json" "DEPS" "*/DEPS" "*.mojom" "*.proto" "*.rs"