.arcconfig

tests: test behavior of IOError during transactions (issue5658)

ui._write(), ui._write_err(), and ui.flush() all trap IOError and
re-raise as error.StdioError. If a caller doesn't catch StdioError
when writing to stdio, it could bubble all the way to dispatch.

This commit adds tests for I/O failures around various transaction
operations.

The most notable badness is during abort. Here, an uncaught StdioError
will result in incomplete transaction rollback, requiring an
`hg rollback` to recover. This can result in a client "corrupting"
a remote repo via terminated HTTP and SSH socket.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}