commit pinned ember-cli version fails

On Windows 10, in an existing Ember project folder with git and clean working tree:

```
C:\Users\edgra\Projects\ember-ide2> ember-cli-update --to 3.12.0
{ Error: Command failed: git --git-dir="C:\Users\edgra\AppData\Local\Temp\tmp-123884j8DXapZNftr\.git" rev-parse vnull
fatal: ambiguous argument 'vnull': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git [...] -- [...]'

at ChildProcess.emit (events.js:214:7)
at maybeClose (internal/child_process.js:925:16)
at Process.ChildProcess._handle.onexit (internal/child_process.js:209:5)
killed: false,
code: 128,
signal: null,
cmd: 'git --git-dir="C:\\Users\\edgra\\AppData\\Local\\Temp\\tmp-123884j8DXapZNftr\\.git" rev-parse vnull',
stdout: 'vnull\n',
stderr: 'fatal: ambiguous argument \'vnull\': unknown revision or path not in the working tree.\nUse \'--\' to separate paths from revisions, like this:\n\'git [...] -- [...]\'\n' }
```

在 Windows 10 上使用现有的 Ember 项目、干净的 Git 工作树和 `ember-cli-update --to 3.12.0` 重现该故障。首先跟踪执行 `git ... rev-parse vnull` 的命令；完成标准是固定版本的 Ember CLI 更新不再因报告的未知修订版本错误而失败。

由索引模型根据 Issue 内容生成。