tweakdefaults: make util.popen4 input buffer size configurable on Windows
This allows us to worry less about deadlocks and be more efficient in our
piped communications (like in fileserverclient.py, for example).
We can also make sure that deadlocks just plainly can't
happen by only writing a known amount of bytes to the pipe.
- does not seem to break any additional tests on Linux
Differential Revision: https://phab.mercurial-scm.org/D1436