[openscreen] Fix some DEPS and PRESUBMIT issues.

1. We weren't checking out the clang-format-diff script, which is used
   by git-cl.
2. buildtools/ should only be checked out when build_with_chromium=False.
3. The license checker is complaining about buildtools/third_party.
   Exclude that path until it can be fixed.

TBR=btolsch@chromium.org

Change-Id: Ibf8d0d989b0d9c1f43016950a9ac87f2b006afbb
Reviewed-on: https://chromium-review.googlesource.com/c/openscreen/+/2928929
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Commit-Queue: mark a. foltz <mfoltz@chromium.org>
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index f8a427d..a07514d 100755
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -199,6 +199,9 @@
     results.extend(_CheckDeps(input_api, output_api))
 
     # Run tools/licenses on code change.
+    # TODO(https://crbug.com/1215335): licenses check is confused by our
+    # buildtools checkout that doesn't actually check out the libraries.
+    licenses.PRUNE_PATHS.add(os.path.join('buildtools', 'third_party'));
     results.extend(_CheckLicenses(input_api, output_api))
 
     return results