Interpreting your question as "find the revision(s) in their repository with the tree for the source they sent you",
ours=$( git rev-parse our_commit_of_their_source^{tree} )
git rev-list --pretty=format:'%h %T %s' | grep $ours
If there aren't any identical trees, you can at least efficiently list the files with differences with git diff-tree --name-status $ours $theirs
to see how many files in a commit are different than yours, it's one way to do what Robin suggested. Maybe do the diff-trees with '-b` to ignore whitespace-only changes.