Allow subversion externals that explicitly include a blank password


-e svn+http://user@repo/path
-e svn+http://user:@repo/path
-e svn+http://user:pass@repo/path

The first and third forms were working as expected (ask for a password, and use "password"), however the middle one wasn't.  This is perfectly valid, but wasn't possible with pip before:

    svn update --username user --password ''

Used for automated deployments, with an svn user that isn't allowed to commit.