Enable loading of hgrc files and normalize diff settings

Not loading hgrc files (the prior behavior) was wrong because hgrc files
could provide desired or even required functionality.

The reason why hgrc files weren't loaded was concerns over consistent
command output, mainly for `hg diff`. These concerns have been addressed
in the form of a custom Mercurial extension that resets diff settings at
run-time.

Fixes bug #3034
Reviewed at https://reviews.reviewboard.org/r/5065/