hgweb: use our forked wsgiheaders module instead of stdlib one
Now we use bytes for headers, rather than native strings.
Differential Revision: https://phab.mercurial-scm.org/D2854
wsgiheaders: import a bytes-ified fork of wsgiref.headers from cpython@46f5072
This will let us restore Python 3 compatibility for tests that do http things.
Differential Revision: https://phab.mercurial-scm.org/D3245
export: enable formatter support (API)
This change is basically the same as "hg cat". A formatter object is created
by caller.
.. api::
``cmdutil.export()`` takes a formatter as an argument.