Tool doesn't work on non git repositories.

ember-cli-update doesn't work on my Mercurial repo:

```
> DEBUG=ember-cli-update,git-diff-apply ember update
ember-cli-update npm info ember-cli versions --json +0ms
ember-cli-update [
ember-cli-update "0.0.0",
ember-cli-update "0.0.1",
ember-cli-update "0.0.2",
ember-cli-update "0.0.3",
ember-cli-update "0.0.4",
ember-cli-update "0.0.5",
ember-cli-update "0.0.6",
ember-cli-update "0.0.7",
ember-cli-update "0.0.8",
ember-cli-update "0.0.9",
ember-cli-update "0.0.10",
ember-cli-update "0.0.11",
ember-cli-update "0.0.12",
ember-cli-update "0.0.13",
ember-cli-update "0.0.14",
ember-cli-update "0.0.15",
ember-cli-update "0.0.16",
ember-cli-update "0.0.18",
ember-cli-update "0.0.19",
ember-cli-update "0.0.20",
ember-cli-update "0.0.21",
ember-cli-update "0.0.22",
ember-cli-update "0.0.23",
ember-cli-update "0.0.24",
ember-cli-update "0.0.25",
ember-cli-update "0.0.27",
ember-cli-update "0.0.28",
ember-cli-update "0.0.29",
ember-cli-update "0.0.32",
ember-cli-update "0.0.33",
ember-cli-update "0.0.34",
ember-cli-update "0.0.35",
ember-cli-update "0.0.36",
ember-cli-update "0.0.37",
ember-cli-update "0.0.38",
ember-cli-update "0.0.39",
ember-cli-update "0.0.40",
ember-cli-update "0.0.41",
ember-cli-update "0.0.42",
ember-cli-update "0.0.43",
ember-cli-update "0.0.44",
ember-cli-update "0.0.45",
ember-cli-update "0.0.46",
ember-cli-update "0.0.47",
ember-cli-update "0.1.0",
ember-cli-update "0.1.1",
ember-cli-update "0.1.2",
ember-cli-update "0.1.3",
ember-cli-update "0.1.4",
ember-cli-update "0.1.5",
ember-cli-update "0.1.6",
ember-cli-update "0.1.7",
ember-cli-update "0.1.8",
ember-cli-update "0.1.9",
ember-cli-update "0.1.10",
ember-cli-update "0.1.11",
ember-cli-update "0.1.12",
ember-cli-update "0.1.13",
ember-cli-update "0.1.14",
ember-cli-update "0.1.15",
ember-cli-update "0.2.0-beta.1",
ember-cli-update "0.2.0",
ember-cli-update "0.2.1",
ember-cli-update "0.2.2",
ember-cli-update "0.2.3",
ember-cli-update "0.2.4",
ember-cli-update "0.2.5",
ember-cli-update "0.2.6",
ember-cli-update "0.2.7",
ember-cli-update "1.13.0",
ember-cli-update "1.13.1",
ember-cli-update "1.13.5",
ember-cli-update "1.13.6",
ember-cli-update "1.13.7",
ember-cli-update "1.13.8",
ember-cli-update "1.13.9",
ember-cli-update "1.13.10",
ember-cli-update "1.13.11",
ember-cli-update "1.13.12",
ember-cli-update "1.13.13",
ember-cli-update "1.13.14",
ember-cli-update "1.13.15",
ember-cli-update "2.2.0-beta.1",
ember-cli-update "2.2.0-beta.2",
ember-cli-update "2.2.0-beta.3",
ember-cli-update "2.2.0-beta.4",
ember-cli-update "2.2.0-beta.5",
ember-cli-update "2.2.0-beta.6",
ember-cli-update "2.3.0-beta.1",
ember-cli-update "2.3.0-beta.2",
ember-cli-update "2.3.0",
ember-cli-update "2.4.0",
ember-cli-update "2.4.1",
ember-cli-update "2.4.2",
ember-cli-update "2.4.3",
ember-cli-update "2.5.0",
ember-cli-update "2.5.1",
ember-cli-update "2.6.0-beta.1",
ember-cli-update "2.6.0-beta.2",
ember-cli-update "2.6.0-beta.3",
ember-cli-update "2.6.0",
ember-cli-update "2.6.1",
ember-cli-update "2.6.2",
ember-cli-update "2.6.3",
ember-cli-update "2.7.0-beta.1",
ember-cli-update "2.7.0-beta.2",
ember-cli-update "2.7.0-beta.3",
ember-cli-update "2.7.0-beta.4",
ember-cli-update "2.7.0-beta.5",
ember-cli-update "2.7.0-beta.6",
ember-cli-update "2.7.0",
ember-cli-update "2.8.0-beta.1",
ember-cli-update "2.8.0-beta.2",
ember-cli-update "2.8.0-beta.3",
ember-cli-update "2.8.0",
ember-cli-update "2.9.0-beta.1",
ember-cli-update "2.9.0-beta.2",
ember-cli-update "2.9.0",
ember-cli-update "2.9.1",
ember-cli-update "2.10.0-beta.1",
ember-cli-update "2.10.0-beta.2",
ember-cli-update "2.10.0",
ember-cli-update "2.10.1",
ember-cli-update "2.11.0-beta.1",
ember-cli-update "2.11.0-beta.2",
ember-cli-update "2.11.0",
ember-cli-update "2.11.1",
ember-cli-update "2.12.0-beta.1",
ember-cli-update "2.12.0-beta.2",
ember-cli-update "2.12.0",
ember-cli-update "2.12.1",
ember-cli-update "2.12.2",
ember-cli-update "2.12.3",
ember-cli-update "2.13.0-beta.1",
ember-cli-update "2.13.0-beta.2",
ember-cli-update "2.13.0-beta.3",
ember-cli-update "2.13.0-beta.4",
ember-cli-update "2.13.0",
ember-cli-update "2.13.1",
ember-cli-update "2.13.2",
ember-cli-update "2.13.3",
ember-cli-update "2.14.0-beta.1",
ember-cli-update "2.14.0-beta.2",
ember-cli-update "2.14.0",
ember-cli-update "2.14.1",
ember-cli-update "2.14.2",
ember-cli-update "2.15.0-beta.1",
ember-cli-update "2.15.0-beta.2",
ember-cli-update "2.15.0",
ember-cli-update "2.15.1",
ember-cli-update "2.16.0-beta.1",
ember-cli-update "2.16.0-beta.2",
ember-cli-update "2.16.0",
ember-cli-update "2.16.1",
ember-cli-update "2.16.2",
ember-cli-update "2.17.0-beta.1",
ember-cli-update "2.17.0-beta.2",
ember-cli-update "2.17.0",
ember-cli-update "2.17.1",
ember-cli-update "2.17.2",
ember-cli-update "2.18.0-beta.1",
ember-cli-update "2.18.0-beta.2",
ember-cli-update "2.18.0",
ember-cli-update "2.18.1",
ember-cli-update "2.18.2",
ember-cli-update "3.0.0-beta.1",
ember-cli-update "3.0.0-beta.2",
ember-cli-update "3.0.0",
ember-cli-update "3.0.1",
ember-cli-update "3.0.2",
ember-cli-update "3.0.3",
ember-cli-update "3.0.4",
ember-cli-update "3.1.0-beta.1",
ember-cli-update "3.1.0",
ember-cli-update "3.1.1",
ember-cli-update "3.1.2",
ember-cli-update "3.1.3",
ember-cli-update "3.1.4",
ember-cli-update "3.2.0-beta.1",
ember-cli-update "3.2.0-beta.2",
ember-cli-update "3.2.0",
ember-cli-update "3.3.0",
ember-cli-update "3.4.0-beta.1"
ember-cli-update ]
ember-cli-update +441ms
ember-cli-update npm info ember-cli@latest version --json +2ms
ember-cli-update "3.3.0"
ember-cli-update +437ms
git-diff-apply git status --porcelain +0ms
fatal: not a git repository (or any parent up to mount point /)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
Not a git repository
```
I think that it should work independently from used SCM (and even without SCM).