tests: work around flaky test failure on FreeBSD and relatives test-http-bad-server has a flaky output case that reproduces fairly often (20% or more of runs on my Mac) on BSD-family OSes. So that tests can be green for the 4.2 release, let's more or less give up on this output, and plan to pursue it in the future.

context: start walking from "introrev" in blockancestors() Previously, calling blockancestors() with a fctx not touching file would sometimes yield this filectx first, instead of the first "block ancestor", because when compared to its parent it may have changes in specified line range despite not touching the file at all. Fixing this by starting the algorithm from the "base" filectx obtained using fctx.introrev() (as done in annotate()). In tests, add a changeset not touching file we want to follow lines of to cover this case. Do this in test-annotate.t for followlines revset tests and in test-hgweb-filelog.t for /log/<rev>/<file>?linerange=<from>:<to> tests.

sshpeer: try harder to snag stderr when stdout closes unexpectedly Resolves test failures on FreeBSD, but I'm not happy about the fix. A previous version of this also wrapped readline by putting the hack in the _call method on doublepipe. That was confusing for readers and wasn't necessary - just doing this on read() is sufficient to fix the bugs I'm observing. We can always come back and do readline later if needed.