py3: only flush before prompting during interactive patch filtering Follows up on c9093ae8d6c4. It's enough to flush just before each prompt.

py3: add a missing b'' prefix in test extension for chg Differential Revision: https://phab.mercurial-scm.org/D7038

fuzz: new target to fuzz jsonescapeu8fast This code just feels complicated enough we should go ahead and give it a dedicated fuzzer: we've found bugs in similar things before. Differential Revision: https://phab.mercurial-scm.org/D7034

fuzz: new fuzzer for fncache-related functions Not all of these are strictly fncache-related, but they all have th same signature and similar-enough behavior that we may as well fuzz them together. No obvious bugs for once, but these felt like they were just complicated enough to cover. Differential Revision: https://phab.mercurial-scm.org/D7033

fuzz: exercise a little more revlog code Differential Revision: https://phab.mercurial-scm.org/D7032

fuzz: new fuzzer for dirs.c This found a six-year-old bug immediately, and then I put it through a few CPU-days of time before sending it. Differential Revision: https://phab.mercurial-scm.org/D7031

dirs: fix trivial over-read of input data This code, introduced in 8c0a7eeda06d, was intentionally over-reading an input string to avoid getting a shared string object for a one-byte input. Unfortunately with an empty input (like in the case of a fuzzer getting started) this was a trivial over-read and triggered an AddressSanitizer failure. I went out of my way to make sure the code still does the copy-avoidance tricks. I don't think this change will cost us much performance since the one-character strings should be cached aggressively anyway. Differential Revision: https://phab.mercurial-scm.org/D7030