It's a bug in 2.0.15 and misconfiguration. By default SVN mirror plugin assumes you have standard trunk/branches/tags structure, and if there're no such directories, no commits are translated. But if you have different repository structure, you can specify it as well, and since version 2.0.16 (when it is released; 2.0.15 doesn't support that) you can specify project root as the only "branch" of the project.
So the steps are:
- Install SVN mirror plugin of version >= 2.0.16 or use this link to 2.0.16-beta3 version (if it not released yet):
http://old.subgit.com/download/stash/stash-svn-importer-2.0.16-beta3.jar
(Choose Administration | Manage add-ons | Upload add-on | From this URL
)
- Create a repository, choose "Subversion Mirror".
- Fill SVN URL, leave Trunk/Branches/Tags/Shelves options as is, you will overwrite them later. Specify other options if you need. Press "Continue".
- Choose Advanced Configuration | Apply changes

Edit the config to remove all trunk/branches/tags/shelves options and to add only
trunk = :refs/heads/master
You can find documentation on the options in SubGit book and in comments in this file starting from #
character.
:refs/heads/master
rule means that SVN path before :
character (which is an empty string, that corresponds to the project root) is mapped to Git reference refs/heads/master
.
- Check authors in "Authors mapping" tab and press "Continue".
That's all, now refs/heads/master
Git reference has the history of your project root in Subversion repository.