allow multiple fallbacks for --affected base branch (#9045)
### Description
We use `main` as our default for base branch, but a lot of repos are
likely using `master`, we should fall back to that if `main` isn't a
valid ref.
### Testing Instructions
see unit tests
https://github.com/vercel/turborepo/pull/9045/files#diff-db1296fb6f1a06e197e7cd6cdf5979b738c6b56b7700b048ab3b00fd767f8aecR734-R746
for situations to test manually.
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Co-authored-by: Chris Olszewski <chris.olszewski@vercel.com>
