Correct optimization from 3464f5e77f34; add a test. If there are two (or more) heads that point to the same .hgtags node, we can safely skip parsing the file in all but the last head. (In 3464f5e77f34, we were parsing the file in the first head and skipping all the others.)

abort: repository a not found!
255
abort: error: Connection refused
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abort: repository a not found!
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abort: destination '../a' already exists
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abort: repository a not found!
255
abort: destination 'q' already exists