Auto merge of #4342 - sid0:hgignore, r=alexcrichton

new: fix hgignore for real

There was an attempt to fix hgignore in #4158, but unfortunately the fix
was incorrect -- hgignore glob syntax is not in fact identical to
gitignore syntax.

To get rooted ignores in Mercurial we must use the regex-based syntax,
so we have no choice but to define a separate set of ignores.