httpclient: update to 54868ef054d2 of httpplus
As of that revision, httpplus fully supports Python 3, including
mimicing all the subtle behavior changes around headers in Python 3's
http.client.
revset: check invalid function syntax "func-name"() explicitly
Before the error was caught at func() as an unknown identifier, and the
optimizer failed to detect the syntax error. This patch introduces getsymbol()
helper to ensure that a string is not allowed as a function name.