R1:aa70b008f3a1

Skip "git for-each-ref" when identifying deleted commits

Summary:
Ref T13647. The ref discovery process prunes commits that no longer exist in the repository before executing "git log <new heads> --not <old heads>" to identify newly published commits.

If we don't do this, the "git log" command will fail if any old head has been pruned from the repository.

Currently, this test for missing commits starts with a call to "git for-each-ref" to attempt to resolve symbols as tag or branch names, but:

- this…