More straightforward way to generate local repository path.

Previously, if you set up two different repos with locations like
/data/hg/nginx-1.2 and /data/hg/nginx-1.3, repo_path for both of them
would be the same: /data/hg/nginx-1.

Now, we use the whole repo URL as the local repo_path, but with
all slashes ('/') replaced by underline characters ('_').