protoci-difference-build - handle large git diffs or build trees

[PeterDSteinberg]

protoci-difference-build needs to handle the cases where a huge number of required builds arise from a git push event, such as:

• Many git diffs in different packages, and/or
• Touching files related to super-node packages, e.g. r, with many downstream packages

One solution for now would be to print structured data, json, with a tag of the packages that need to be built but didn't. In the event that a super node like r is touched, the protoci-difference-build could identify some r related trees of packages it can build in the allotted time, then print out something like:

TESTING_FOLLOW_UP {"require_testing": ["r-essentials", "r-maps"]}

to mention that those two r packages should be tested, but the build worker didn't get to it.

[PeterDSteinberg]

This commit to master aims to solve this problem by offering an argument to protoci-difference-build that is the -depth to which the package tree should be searched for packages that are downstream-affected by a git diff'ed package. For example, a seaborn test should be triggered if -depth > 0 when matplotlib is part of the git diff.