Switch the Mercurial support to use the command line tool.

Our Mercurial support interfaced with the 'hg' module to cat files in a
repository, but this can easily be accomplished by the command line
tool. That tool is the correct API to use, not the module. More
importantly, it resolves a licensing issue due to the 'hg' module being
GPL, which was a terrible oversight that we want to address. This will
be backported to prior releases.
Switch over to using a real hg repo instead of a bundle.

The 'hg' command-line tool doesn't entirely work when treating a .bundle file
as if it were a repo. This change extracts the bundle into a new hg repo
directory, and changes our test suite over to use it.

Testing done: Ran unit tests.
Merge branch 'release-1.6.x' into release-1.7.x
Fix some format strings for localization.

Some format strings for SCMErrors were formatting multiple values without using
named formatters, which makes it impossible for translators to reorder the
sentence.

Testing done:
* Ran unit tests
* Extracted message catalogs and didn't see any errors.

Reviewed at https://reviews.reviewboard.org/r/5068/
Rename "Add Comment" to "New Comment"

Short of a redesign of the comment dialog to clear up the issues with replying
to existing comments vs. creating new ones, renaming this may help with some of
the confusion.

Suggested at bug 3160.
Merge branch 'release-1.7.x'