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>