ensure files are marked readonly after unlocking by ID

[technoweenie]

Fixes #2640 by moving the file permission logic from UnlockFile() to UnlockFileById(). The path is taken from the lock property returned by the /unlock api endpoint.

$ GIT_CURL_VERBOSE=1 git lfs unlock --id=123
> POST /my-repo.git/info/lfs/locks/123/unlock HTTP/1.1
> Host: github.com
> Accept: application/vnd.git-lfs+json; charset=utf-8
> Authorization: Basic * * * * *
> Content-Length: 15
> Content-Type: application/vnd.git-lfs+json; charset=utf-8
> User-Agent: git-lfs/2.3.1 (GitHub; darwin amd64; go 1.9)
>
{"force":false}

< HTTP/1.1 200 OK
< Content-Length: 141
< Content-Type: application/vnd.git-lfs+json
< Date: Tue, 03 Oct 2017 17:11:25 GMT
< X-Frame-Options: DENY
< X-Github-Request-Id: FF7E:5D17:5AA5486:AB0C1BC:59D3C4BD
<
{"lock":{"id":"123","path":"my-file.zip","locked_at":"2017-10-03T17:11:09Z","unlocked_at":"2017-10-03T17:11:25.692648804Z"}}
Unlocked Lock 123