Baseline diff (`--baseline-diff`)

From #109

```py
a: int = ""
```
`mypy --write-baseline a.py`

```py
b: str = 1
```
`mypy --write-baseline --baseline-file b.json a.py`

```
mypy --baseline-diff b.json

Removed:
a.py:1 can't assign a str to a variable of type int

Added:
a.py:1 can't assign an int to a variable of type str
```

Start by reproducing the two baseline-writing commands with a.py, then run `mypy --baseline-diff b.json` against the changed diagnostics. Trace the baseline and command-line entry points involved in these commands. Done means the output identifies diagnostics removed from and added to the baseline, matching the shown example.

Written by the indexing model from the issue text.