eslint.fix flag does not commit changes

[RainNapper]

When there are autofixable lint errors detected by eslint, the order of operations is: 1) arc diff 2) if there are uncommited changes, amend or commit 3) arc lint, which autofixes and returns successful 4) autofixed changes are not committed nor staged 5) diff is pushed to phabricator without changes

This means that arc diff fails to properly apply lint changes before creating a diff.

See messages for a file provided by eslint@6.8.0

        "messages": [
            {
                "ruleId": "prettier/prettier",
                "severity": 2,
                "message": "Replace `(flow.component·&&·flow.component.archived)` with `flow.component·&&·flow.component.archived`",
                "line": 61,
                "column": 10,
                "nodeType": null,
                "messageId": "replace",
                "endLine": 61,
                "endColumn": 53,
                "fix": {
                    "range": [
                        1462,
                        1505
                    ],
                    "text": "flow.component && flow.component.archived"
                }
            },
            {
                "ruleId": "prettier/prettier",
                "severity": 2,
                "message": "Insert `(⏎··········`",
                "line": 65,
                "column": 13,
                "nodeType": null,
                "messageId": "insert",
                "endLine": 65,
                "endColumn": 13,
                "fix": {
                    "range": [
                        1675,
                        1675
                    ],
                    "text": "(\n          "
                }
            },
            {
                "ruleId": "prettier/prettier",
                "severity": 2,
                "message": "Replace `⏎⏎⏎········` with `········)`",
                "line": 66,
                "column": 1,
                "nodeType": null,
                "messageId": "replace",
                "endLine": 69,
                "endColumn": 9,
                "fix": {
                    "range": [
                        1685,
                        1696
                    ],
                    "text": "        )"
                }
            }
        ],

[jparise]

Fixed by #79