hook: write hook output to ui fout descriptor

output on stderr will also be written to ui.fout, unless sys.__stdout__
is passed in (see util.system), thus not changing previous behavior.

This fixes a bug where hooks run through the command server would mess up
with the command protocol, sending non-channeled data to the client.

mercurial/hook.py

@@ -107,7 +107,7 b' def _exthook(ui, repo, name, cmd, args, '
     if 'HG_URL' in env and env['HG_URL'].startswith('remote:http'):
         r = util.system(cmd, environ=env, cwd=cwd, out=ui)
     else:
-        r = util.system(cmd, environ=env, cwd=cwd)
+        r = util.system(cmd, environ=env, cwd=cwd, out=ui.fout)
     if r:
         desc, r = util.explainexit(r)
         if throw: