hgweb: allow defining Server response header for HTTP server By default, the string contains the Python version. Server operators may want to change this. Because we like deterministic tests, we change the test harness to always set this value to a known string. Various globs over the server header have now been removed. # no-check-commit because we add version_string() Differential Revision: https://phab.mercurial-scm.org/D2840