command/pr, command/list: When the latest commit is not mergeable, tctest should return an error

[gdavison]

If the latest commit is not mergeable, tctest will identify tests from the latest mergeable commit. It would be helpful if the pr and list commands would return an error stating that the current PR is not mergeable.

A flag to override this behaviour should be added, which would emit a warning and continue running the command.

Steps to reproduce:

1. On a project with status checks configured, introduce an error, e.g. a linting error
2. Run tctest pr
3. Observe TeamCity running old tests

Proposed behaviour:

$ tctest pr 12345
Discovering tests for pr #12345 (...)
ERROR: PR #12345 is not mergeable

$tctest pr 12345 --ignore-mergeability
Discovering tests for pr #12345 (...)
WARNING: PR #12345 is not mergeable, using last mergeable commit ab3410
...