Write out hgrc properly. Previously, we simply appended to the hgrc file, which meant that it ended up containing multiple "paths" sections. Now, we only modify "paths.default".

\.elc$
\.orig$
\.rej$
~$
\.so$
\.pyc$
\.swp$
\.prof$
^tests/.*\.err$
^build/
^dist/
^doc/.*\.[0-9](\.(x|ht)ml)?$
^MANIFEST$
^\.pc/
^patches/
^mercurial/__version__.py$