Add upstream sync script.

Helper script to setup clone to sync master branch with an upstream remote.
Default setup to keep a fork in sync with the github public clone. Very handy
for tracking the master in a github clone that can then be used to stage
working branches and submit pull requests.
fix yaml_emit_file function
Merge branch 'pull-request/2'
Fetch pull requests from php mirror

The pull-request.sh utility script was incorrectly set to fetch pull requests
made against a clone repository instead of the canonical PHP mirror.
Merge branch 'pull-request/1'

Enable yaml_emit_file() which has apparently been marked as unimplemented for
years even though all the code was available in the source.