Write a config option if we protect master
Write a config option if we protect master

Summary:
This will allow other tools to check that option to decide whether to
operate.

Test Plan:
```
$ git config --local kaclone.protect-master
$ ka-clone --repair --protect-master
[output omitted]
$ git config --local kaclone.protect-master
true
```

Reviewers: csilvers

Reviewed By: csilvers

Subscribers: slim

Differential Revision: https://phabricator.khanacademy.org/D50376

Khan/ka-clone,Khan/ka-clone