IOW, you can use this thing to look for likely duplicate commits.
+When dealing with git-diff-tree output, it takes advantage of
+the fact that the patch is prefixed with the object name of the
+commit, and outputs two 40-byte hexadecimal string. The first
+string is the patch ID, and the second string is the commit ID.
+This can be used to make a mapping from patch ID to commit ID.