Getting the difference between two repositories

In repo_a:

git remote add -f b path/to/repo_b.git
git remote update
git diff master remotes/b/master
git remote rm b

Meld can compare directories:

meld directory1 directory2

Just use the directories of the two git repos and you will get a nice graphical comparison:

When you click on one of the blue items, you can see what changed.

You can add other repo first as a remote to your current repo:

git remote add other_name PATH_TO_OTHER_REPO

then fetch brach from that remote:

git fetch other_name branch_name:branch_name

this creates that branch as a new branch in your current repo, then you can diff that branch with any of your branches, for example, to compare current branch against new branch(branch_name):

git diff branch_name

Once you have both branches in one repository you can do a git diff. And getting them in one repository is as easy as

git fetch /the/other/repo/.git refs/heads/*:refs/remotes/other/*