sparse-revlog: fix delta validity computation When considering the validity of a delta with sparse-revlog, we check the size of the largest read. To do so, we use some regular logic with the extra delta information. Some of this logic was not handling this extra delta properly, confusing it with "nullrev". This confusion with nullrev lead to wrong results for this computation but preventing a crash. Changeset 781b2720d2ac on default revealed this error, crashing. This changeset fixes the logic on stable so that the computation is correct (and the crash is averted). The fix is made on stable as this will impact 4.7 clients interacting with sparse-revlog repositories (eg: created by later version).

convert: don't drop missing or corrupt tag entries Cleaning up the tags file could be a useful feature in some cases, so maybe there should be a switch for this. However, the default hg -> hg convert tries to maintain identical hashes (thus convert.hg.saverev is off by default, but is on by default for other source types). It looks like _rewritesubstate() has a `continue` in it, and therefore a similar problem. I ran into this conversion divergence when a coworker "merged" two repositories by copy/pasting all of the files from the source repo and massaging the code, and forgetting to revert the .hg* files. That silently emptied the .hgtags file after the conversion. (This isn't the manifest node bug Yuya has been helping with- this occurred well after the bzr -> hg conversion and wasn't a merge commit, which made it extra puzzling. That bug is still an issue.)