resolve_relative_external_url should be public

[Stefan Fuhrman <st...@alice-dsl.de>]

I'm currently improving TSVN's repository browsing capabilities.
We already blend svn:externals definitions with ordinary folder
content and allow users to follow them transparently. This requires
the absolute URLs to be known.

However, there seems to be no public interface to convert
relative URLs found in svn:externals to absolute ones. Therefore,
I have to replicate the implementation of said function in TSVN.

So, it would be nice just to call some SVN lib function instead.
I already happy with the function signature but other clients
might want to specify parent_dir_url only as repos_root_url
is redundant, IMHO.

Cheers!
Stefan^2.