replay: Use remote repository base URL to export external located in it
Some externals might be located in the same repository we are currently loading. In that case, replace the external base URL which corresponds to the origin URL by the remote repository URL.
When we use SvnLoaderFromRemoteDump, that remote repository URL corresponds to a local repository mounted from a dump file so we can export the external path in a much faster way without any costly network requests.
Related to #611
Migrated from D7099 (view on Phabricator)