Add bootstrap debugging and ability to bootstrap current directory

[kpaulisse]

Overview

This pull request adds two new options:

• --debug-bootstrap: In conjunction with --debug will print the STDERR and STDOUT from the bootstrap script in the debugging output
• --bootstrap-current: When the to-branch is unspecified and the current directory is used for one of the branches, the bootstrap script normally does not execute (as it's assumed you've already bootstrapped your development directory). If you want the bootstrap script to execute, this option will create a copy of the current directory and then run the bootstrap script there.

Checklist

• Make sure that all of the tests pass, and fix any that don't. Just run rake in your checkout directory, or review the CI job triggered whenever you push to a pull request.
• Make sure that there is 100% test coverage by running rake coverage:spec or ignoring untestable sections of code with # :nocov comments. If you need help getting to 100% coverage please ask; however, don't just submit code with no tests.
• If you have added a new command line option, we would greatly appreciate a corresponding integration test that exercises it from start to finish. This is optional but recommended.
• If you have added any new gem dependencies, make sure those gems are licensed under the MIT or Apache 2.0 license. We cannot add any dependencies on gems licensed under GPL.
• If you have added any new gem dependencies, make sure you've checked in a copy of the .gem file into the vendor/cache directory.

/cc #4