First of all, get the git bundle.

newtricks:~ ted$ cd ~/Library/Application\ Support/TextMate/Bundles
newtricks:Bundles ted$ git clone git://gitorious.org/git-tmbundle/mainline.git Git.tmbundle
Initialized empty Git repository in /Users/ted/Library/Application Support/TextMate/Bundles/Git.tmbundle/.git/
remote: Counting objects: 5018, done.
remote: Compressing objects: 100% (1612/1612), done.
remote: Total 5018 (delta 3342), reused 4957 (delta 3303)
Receiving objects: 100% (5018/5018), 634.79 KiB | 181 KiB/s, done.
Resolving deltas: 100% (3342/3342), done.
newtricks:Bundles ted$

Then, invoke Textmate and navigate to Bundles->Git->Administration ->Update Git Bundle. The following screen appears.

Here are some references.

http://gitorious.org/git-tmbundle
http://blog.macromates.com/2008/git-bundle/

This Textmate git bundle seems to work pretty well. Here's how I tried it out. First of all use Textmate to open a file that is a member of a git repository. Make a change and save it. Then, from the Textmate window do a Bundles -> Git -> Commit.

Double click on the filename in the Choose files to commit pane. A Textmate window opens up that shows the file in a diff format.

Write a description in the Summary of changes pane. Click commit. The window with the Summary and Commit button disappears and the other window shows what happened.

I still have the two Textmate windows open. Close the one with the diff and select the one with the file. Then, from the Textmate menu do a Bundles -> Git -> Log, and the following window appears.