tests: extract the http server authentication extension to a single module
We had 4 copy/pastes of this, and no coverage for http digests (which are
currently broken on py3).
subrepo: avoid false unsafe path detection on Windows
Subrepo paths are not normalized for the OS, so what was happening in the
subsequent root path check was:
root -> $TESTTMP\
issue1852a\sub/repo
util.expandpath(...) -> $TESTTMP\
issue1852a\sub/repo
os.path.realpath(...) -> $TESTTMP\
issue1852a\sub\repo