Truncating mypy_primer output per project

[JukkaL]

If there is a lot of mypy_primer output from a single project, the mypy primer is truncated so that output from some projects may not be visible at all. It could be better if we'd separately truncate the output of each project. We could still have a global output limit that is, for example, 3x the per-project output limit, to avoid huge output if a change breaks everything.

[JukkaL]

Example PR where this could have been helpful: #14041