Diff between the non-head git work dir and non-git kernel sources

Hi

I am sorry if this a newbie question, but I have tried to read howtos from the web without finding clear documentation or howto to following kind of problem.

I have checked out the omap git tree with command

    git-clone rsync://source.mvista.com/git/linux-omap-2.6.git 
linux-omap-2.6

and now I am obviously having the omap kernel head in the linux-omap-2.6 working directory + hidden ".git" with commit history. In addition I have my own "non git" development tree that is made on top of traditional kernel snapshots. I however know that the sources in this dir reflects to state in git history when following patch was committed

    
http://source.mvista.com/git/gitweb.cgi?p=linux-omap-2.6.git;a=commitdiff;h=d24aff0bd3e788d69a45a9d1b1eecda88d847a41

I would now like to revert the sources in git working dir to state that was after this d24aff0bd3e788d69a45a9d1b1eecda88d847a41 commit and then make the diff between that kernel source version and my non-git source version and I am now trying to find out what is the "correct" way of doing this.

1) If understood the documentation correctly I could move the working 
dir to contain the state after
d24aff0bd3e788d69a45a9d1b1eecda88d847a41 commit in a following way
a) open qgit and mark correct commit with "make tag mytag" command
b) change the files in working dir to be same than during the "mytag" 
commit time by using command git checkout -f mytag
   
Are there simpler method for doing this that uses commit id 
d24aff0bd3e788d69a45a9d1b1eecda88d847a41 directly without need to make 
tag or branch?

2) Does git-diff support making the diff between git's working dir and 
"my non" git kernel source dir.
Or should I just just use normal diff and order that to ignore .git dir 
and .gitignore files?

Mika