Q:

Download a specific tag from a different respository

On an old project I worked on, I have a Drupal set up that is established and set to version 6.20. Now that I know about Git and such, I have uploaded my system to bitbucket, but at the same time, I want to update the site to 6.33 but I want to do it through a "git pull" but from the main Drupal repository on github.
So, seeing as how my git repo is set to my private bitbucket repository, how do I do a download of just Tag 6.33 from the Drupal github repo?
After this, I plan to do a step-by-step update of all the files between 6.33 and my current site.
(The reason for this is I had to lightly modify a Drupal system file and want to make sure what I modified doesn't get overwritten. I am doing this all over commandline too, so any git commandline diff tools would help!)

A:

I'm presuming you created your original project from a tarball of 6.20 or something, and that you don't have project history going back?  Given that I would suggest the following 
Just do a clone of the drupal github repo ( I don't know the exact syntax here, something like ) 
git clone https://github.com/drupal/drupal

Then you would want to check out from the old 6.20 tag (guessing on tag syntax too, use whatever git tag -l reports) and create a branch off of that.
git checkout -b my_stuff 6.20

Now use filesystem commands to overwrite everything in your working directory with what you have, and commit the result.  (note that if you do have history of your project, you would want to import it at this same location probably, but create multiple commits).  
Once you have done that, you would do 
git merge 6.33

(or use rebase if you'd rather).

