Fix bugs in lock diff

[psakievich]

The lock-diff command came out of a project that was using it in a specific way. Several bugs have been found from more general use.

Outstanding issues:

• Multiple specs: this tool was mainly written for unified env's but compilers as deps is leading to a lot more repeated specs in the graph. Need better logic/generalization for diffing the graph
• Testing: add some basic unit-tests so this can be expanded