lfs: narrow the exceptions that trigger a transfer retry The retries were added to workaround TCP RESETs in fb-experimental fc8c131314a9. I have no idea if that's been debugged yet, but this wide net caught local I/O errors, bad hostnames and other things that shouldn't be retried. The next patch will validate objects as they are uploaded, and there's no need to retry those errors. The spec[1] does mention that certain http errors can be retried, including 500. But let's work through the corruption detection issues first. [1] https://github.com/git-lfs/git-lfs/blob/master/docs/api/batch.md

test-lfs: add tests around corrupted lfs objects These are mostly tests against file:// based remote stores, because that's what we have the most control over. The test uploading a corrupt blob to lfs-test-server demonstrates an overly broad exception handler in the retry loop. A corrupt blob is actually transferred in a download, but eventually caught when it is accessed (only after it leaves the corrupt file in a couple places locally). I don't think we want to trust random 3rd party implementations, and this would be a problem if there were a `debuglfsdownload` command that simply cached the files. And given the cryptic errors, we should probably validate the file hash locally before uploading, and also after downloading.

lfs: add note messages indicating what store holds the lfs blob The following corruption related patches were written prior to adding the user level cache, and it took awhile to track down why the tests changed. (It generally made things more resilient.) But I think this will be useful to the end user as well. I didn't make it --debug level, because there can be a ton of info coming out of clone/push/pull --debug. The pointers are sorted for test stability. I opted for ui.note() instead of checking ui.verbose and then using ui.write() for convenience, but I see most of this extension does the latter. I have no idea what the preferred form is.

tests: teach `f` to handle sha256 checksums