The git plugin is included in the newest mingle installer. Beginning with Mingle 3.3, git support is included with Mingle. And thus it is officially supported.
Initially we had the community contribution git plugin, http://community.thoughtworks.com/posts/5384ef23b1is This one was not production ready.
Now we have an official plugin developed by ThoughtWorks Studios which is included with the Mingle Installer.
Although all our SCM integrations are plugins, some are included in the Mingle download.