hgweb: move HTTP content types out of header templates
This removes the ability for templates to add custom HTTP headers, which can
easily be re-added if someone needs it. Thanks to asak for repeatedly reviewing
this patch and helping to iron out the quirks.
% file replaced with directory
adding a
% should fail - would corrupt dirstate
abort: file 'a' in dirstate clashes with 'a/a'
% directory replaced with file
adding a/a
% should fail - would corrupt dirstate
abort: directory 'a' already in dirstate
% directory replaced with file
adding b/c/d
% should fail - would corrupt dirstate
abort: directory 'b' already in dirstate