revlog: detect incomplete revlog reads _readsegment() is supposed to return N bytes of revlog revision data starting at a file offset. Surprisingly, its behavior before this patch never verified that it actually read and returned N bytes! Instead, it would perform the read(), then return whatever data was available. And even more surprisingly, nothing in the call chain appears to have been validating that it received all the data it was expecting. This behavior could lead to partial or incomplete revision chunks being operated on. This could result in e.g. cached deltas being applied against incomplete base revisions. The delta application process would happily perform this operation. Only hash verification would detect the corruption and save us. This commit changes the behavior of raw revlog reading to validate that we actually read() the number of bytes that were requested. We will raise a more specific error faster, rather than possibly have it go undetected or manifest later in the call stack, at delta application or hash verification. Differential Revision: https://phab.mercurial-scm.org/D5266