Empty directories are not converted

[alexgit2k]

Git does not allow adding empty directories (see https://git.wiki.kernel.org/index.php/GitFaq#Can_I_add_empty_directories.3F). As this is possible in SVN it could break some repositories as it is in our case.

Therefore svn2git should add an empty .gitignore-file if it recognizes an empty directory, before converting it to git.