When migrating Subversion (SVN) to Git, the default tool to use is git-svn which is included in the Git distribution. That’s nice when directly switching over to Git, as you only need a one-time repository conversion.
In cases when you need a step by step migration you will have a mixed toolstack (Some developers use Subversion clients, others use Git). In this case, you’ll have massive limitations in the Git features that are allowed to use (e.g. you need to have a linear commit history). That’s not much fun …
With the commercial tool SubGit you can hide that mixed toolstack on the server-side.
Every developer can use the matching server for his client (Subversion or Git), while SubGit cares about the synchronization of both server-side repositories.
On the SubGit website, you can have a look at a comparision between git-svn and SubGit.
TMate Software, the company behind SubGit also provides a plugin for Atlassian‘s Git Repository Management Server Stash. This plugin provides support for importing Subversion repositories into Git Repositories hosted in Stash. You can have a look at it at the SubGit Website or the Atlassian Marketplace.