Merge branch 'jk/pipe-command-nonblock'

Merge branch 'jk/pipe-command-nonblock'

Fix deadlocks between main Git process and subprocess spawned via
the pipe_command() API, that can kill "git add -p" that was
reimplemented in C recently.

* jk/pipe-command-nonblock:
  pipe_command(): mark stdin descriptor as non-blocking
  pipe_command(): handle ENOSPC when writing to a pipe
  pipe_command(): avoid xwrite() for writing to pipe
  git-compat-util: make MAX_IO_SIZE define globally available
  nonblock: support Windows
  compat: add function to enable nonblocking pipes