user1 has checked out source code under ~/proj1 using his ssh keys stored under ~/.ssh
user2 would like to do a "git pull" to synchronize the changes under ~/proj1 and he would have to do this using his ssh keys.

user2 modified ~/proj1/.git/config to reflect url as user2@host:/foo/proj1.git
What would be the procedure to accomplish a git pull or a subsequent push using user2's credentials?