verify: allow the storage to signal when renames can be tested on `skipread` This applies the new marker in the lfs handler to show it in action, and adds the test mentioned at the beginning of the series to show that fulltext isn't necessary in the LFS case. The existing `skipread` isn't enough, because it is also set if an error occurs reading the revlog data, or the data is censored. It could probably be cleared, but then it technically violates the interface contract. That wouldn't matter for the existing verify algorithm, but it isn't clear how that will change as alternate storage support is added. The flag is probably pretty revlog specific, given the comments in verify.py. But there's already filelog specific stuff in there and I'm not sure what future storage will bring, so I don't want to over-engineer this. Likewise, I'm not sure that we want the verify method for each storage type to completely drive the bus when it comes to detecting renames, so I don't want to go down the rabbithole of having verifyintegrity() return metadata hints at this point. Differential Revision: https://phab.mercurial-scm.org/D7713