debugcommands: create new debugantivirusrunning command This writes the EICAR test file to .hg/cache, in an attempt to trigger an AV scanner's scanning engine. This should let us (in theory) detect some cases when a user's slowness is a result of AV scanning. Differential Revision: https://phab.mercurial-scm.org/D8353

windows: always work around EINVAL in case of broken pipe for stdout / stderr In 29a905fe23ae, I missed the fact that the `winstdout` class works around two unrelated bugs (size limit when writing to consoles and EINVAL in case of broken pipe) and that the latter bug happens even when no console is involved. When writing a test for this, I realized that the same problem applies to stderr, so I applied the workaround for EINVAL to both stdout and stderr. The size limit is worked around in the same case as before (consoles on Windows on Python 2). For that, I changed the `winstdout` class.

tests: make pipes / PTYs non-inheritable in test-stdio.py A following patch requires that to test closing the receiving end of the pipe / PTYs. Even for existing tests, it might be safer to make the lifetime of the pipes / PTYs as short as possible.

util: enhance `nullcontextmanager` to be able to return __enter__ result This makes its functionality and signature equivalent to Python 3.7’s contextlib.nullcontext().