Prevent possible recursion limit issue in migration planning

[bwreilly]

This switches out the recursive depth first search for stack based approach.

Additionally adds test that replicates original potential problem.

[collinanderson]

If possible, please separate out the whitespace changes from the actual code changes. Also, patches should generally be against the master branch (instead of 1.7.x)

[bwreilly]

Going to do this against master, and remove whitespace changes.