Here is what the above code is Doing:
1. git diff-tree -r bd61ad98
4. -r bd61ad98
The first line tells git to show a diff between the bd61ad98 commit and its parent.
The second line tells git not to show the SHA1 of the commit.
The third line tells git to show only the names of the files that were changed.
The fourth line tells git to use the bd61ad98 commit.