default compare function bug

[mixmix]

function cmp (a, b) {
  return a < b ? -1 : a > b ? 1 : 0
}

merge seems to choke if the return value is 0

solution change the default to fall back to e.g. 1 (or oscillate between 1/ -1)

[mixmix]

Oh I went to write a test for this and in reading the source code found it was dropping values (which was a surprise!)

https://github.com/pull-stream/pull-merge/pull/3/files
This PR makes the README clearer

[regular]

@mixmix is this why we also have pull-merge-sorted? I just had a long debug session revealing that this module was to blame (one source stream at some point was ignored forever, without compare ever returning 0). Switching to merge-sorted fixed it. Wasn't able to isolate the problem yet or make a test. Just wanted to share in case it helps someone out there.