Here's how. 

1. Jail break your ipad. 

2. Download Terminal and Backgrounder from cydia.  (Terminal is almost impossible to use without backgrounder)

3. Open the terminal.  In the root directory, you will see a folder called Applications.  All of your apps are stored in there. If you navigate into applications, you'll see a bunch of folders named with alphanumeric strings.  One of these folders is textastic, so use the command 'find -iname textastic' to find out which folder textastic is in.  Navigate to that folder.  

4.  In the textastic folder is a folder called Documents.  This folder contains of the documents you wrote in textastic, and has the same directory structure as your textastic app.  Go into Documents.  Navigate to the folder that you want to git.  

5.  Type git init to initialize your git repository in that folder

6.  Whenever you want to commit, switch over to the terminal and make your commit, then go back to happily coding in textastic. 

Congrats.  You are using git with textastic. 

If you open up that folder in textastic, you will notice that there is a .git folder.  

I have been using this setup to git my textastic projects without causing any problems with the textastic app.