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`git diff-tree` should be used with the `-r` flag if we want to recurse
into subtrees (e.g. to check if css/css-animations is affected by a
commit as opposed to just css/). This is the only place where we forgot
to do that and it has caused some false positives of "affecting this
directory" in our bug filing.

Fixed: 850088
Change-Id: Ifdd1b9eeba2c5749effdab1041d86c6500160ce7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2590014
Commit-Queue: Robert Ma <robertma@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Auto-Submit: Robert Ma <robertma@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836670}