The TestResolver class uses self.vcs.get_outgoing_files('AM')  to determine which files have been added or modified when the "what" argument to "./mach test" is empty. This falls back to the IMPACTED_TESTS annotations and is meant to run the tests associated with the modified files. Steps to reproduce are to inspect the return value of this line. In a git repository it is always an empty array.  https://searchfox.org/mozilla-central/rev/0bc3300a3abc4b12f14bd0ddf77ce8aa8125043c/testing/mozbase/moztest/moztest/resolve.py#412
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