Fixes T2461. Similarly to arc lint --everything, arc unit --everything should work without a base commit.
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- rARC0b22838ccaa8: Allow `arc unit --everything` to work without a base commit
Removed the .git/arc/default-relative-commit file and ran arc unit --everything.
This feels a little wasteful since engines may be able to avoid doing this work (e.g., Phutil engines can enumerate tests without scanning the disk for files) but we can deal with that when we, e.g., adjust APIs for .arcunit or whatever.