Select a minimal number of owners for a set of files

This CL adds a function which takes a set of files, and attempts to
select a single owner for all of them. If it cannot, it falls back to
the standard owner selection algorithm, which may result in more owners
being chosen than necessary, but guarantees that a valid set of owners
is always returned.

Bug: 389069356
Change-Id: I985804040f149a02bfb5b3c6b946602a81334e7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6321289
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Devon Loehr <dloehr@google.com>

tests/owners_client_test.py

@@ -244,6 +244,33 @@ class OwnersClientTest(unittest.TestCase):
             self.client.BatchListOwners(
                 ['bar/everyone/foo.txt', 'bar/everyone/bar.txt', 'bar/foo/']))
 
+    def testSuggestMinimalOwners(self):
+        self.client.owners_by_path = {
+            'bar/everyone/foo.txt': [alice, bob, emily],
+            'bar/everyone/bar.txt': [bob, emily],
+            'bar/foo/': [bob, chris, emily],
+            'baz/baz/baz': [chris, dave, emily]
+        }
+
+        self.assertEqual([bob],
+                         self.client.SuggestMinimalOwners([
+                             'bar/everyone/foo.txt', 'bar/everyone/bar.txt',
+                             'bar/foo/'
+                         ]))
+
+        self.assertEqual([chris],
+                         self.client.SuggestMinimalOwners(
+                             ['bar/foo/', 'baz/baz/baz']))
+
+        # If no common owner exists, fallback to returning multiple owners
+        self.assertEqual([alice, bob, chris],
+                         self.client.SuggestMinimalOwners([
+                             'bar/everyone/foo.txt', 'bar/everyone/bar.txt',
+                             'bar/foo/', 'baz/baz/baz'
+                         ],
+                                                          exclude=[emily]))
+
+
 
 class GetCodeOwnersClientTest(unittest.TestCase):
     def setUp(self):