svn_repo: Fix root directory computation in case of scheme redirection
requested to merge anlambert/swh-loader-svn:root-directory-computation-handle-redirection into master
When a repository URL gets scheme redirected, typically from using HTTP to HTTPS, ensure proper computation of the repository root directory to avoid loading issues afterwards.
Fix the loading of http://svn.mikrokopter.de/mikrosvn/Projects/ referenced in SWH-LOADER-SVN-GW, other referenced origins (texlive ones) cannot be loaded as svnrdump ends up with the following error: svnrdump: E210008: Error while replaying commit.