ListOwners returns all OWNERS for a file, which is undesirable in cases
when only direct OWNERS are needed.

Bug: 1351212, 1351519
Change-Id: I693b6645c780aa589e8ab24d0b58691f4aeb30f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3823299Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>