Fix fetching commits by branch on GitHub.

When I fixed filtering of SVN commits by branch, several versions
ago, I broke filtering by branch on GitHub.

The New Review Request page was no longer passing the starting SHA,
which GitHub needed, resulting in all commits being returned. This call
has been fixed.

It's worth noting that the GitHub API allows for filtering by branch,
but you can't both use that and SHAs for pagination. The SHA is good
enough, if provided, so we'll continue to use that, falling back on a
branch if only a branch is provided. The code has been updated with
documentation to cover this.

Reviewed at https://reviews.reviewboard.org/r/6575/
Remove an old comment about Repository path:local_site constraints.

We had an old comment describing how a path:local_site unique constraint
was causing problems when archiving repositories. This was an issue
because an archived repository would keep the path, but wouldn't be
shown in the UI, resulting in a conflict when trying to add a new
repository with the same path.

We fixed this in 2.0 by including an archive timestamp in the key. The
comment hasn't been relevant for a while now, so we can safely delete
it.
Fix generating the path to the manual URL for dev builds.

The old manual URL generation function was assuming that any
non-released builds should us the 'dev' docs, but this is often wrong.
What we actually want is to use the 'dev' docs for any X.Y.0 releases
that aren't marked as "final". This changes the logic accordingly.

Reviewed at https://reviews.reviewboard.org/r/6579/