UPDATE – See comment #7 from Matt Parnell.  His method seems to be a more elegant solution.  You can set a shell variable for the git path within textmate.

I had some funky stuff going on in textmate when I was trying to search my project for a particular string. Textmate kept crashing, and then it gave me the option to reset my preferences (or something like that), so I chose “yes”. After I did this, everything seemed to be working fine except for Git. I kept getting this error:

textmate sh: git: command not found

Did a bit of searching and it appears that textmate was not able to see Git in its path. So the solution is to add a symbolic link in /usr/bin which points to the executable in /usr/local/git/bin/git
Here is what you need to do:

>cd /usr/bin
>ln /usr/local/git/bin/git git

Basically, this just adds a link to the git executable and Textmate is able to find it becuase /usr/bin is in its path.