buildmeta: verify uuid when passed explicit url 2c87bdc43d3c changed buildmeta to read the uuid from local disk, if it's available. As a side effect, it disabled the validation of the uuid we've recorded locall in the commits against the uuid of the repo we are building against. This is probably reasonable when someone runs a bare updatemeta or rebuildmeta. I think it's a worthwhile sanity check when someone passes an explicit repository url to rebuild/updatemeta. This restores the validation in that case, and fixes a failing test in the process.

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