debugwireproto: close the write end before consuming all available data
And make it read all available data deterministically. Otherwise util.poll()
may deadlock because both stdout and stderr could have no data.
Spotted by the next patch which removes stderr from the fds.
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https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
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commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).