Error getting line diffs for newly added files

[50Wliu]

Similar to #93.

Steps to reproduce:

1. Open dev tools
2. Move a file from a subfolder into the root folder using Atom's Tree View (this step might be overly specific, not sure)
3. Errors occur
   OR
4. Open dev tools
5. Create a new file using Tree View's "Add File" feature
6. Errors occur

Once again, two errors:

Error getting line diffs for C:\Users\username\Documents\GitHub\Three\style.css:
(anonymous function)    @   C:\Program Files\Atom\resources\app.asar\node_modules\git-diff\lib\git-diff-view.js:166
tryCallOne  @   C:\Program Files\Atom\resources\app.asar\node_modules\nodegit\node_modules\promisify-node\node_modu…:37
(anonymous function)    @   C:\Program Files\Atom\resources\app.asar\node_modules\nodegit\node_modules\promisify-node\node_modu…:103
flush   @   C:\Program Files\Atom\resources\app.asar\node_modules\nodegit\node_modules\promisify-node\node_modu…:50
doNTCallback0   @   node.js:443
_tickCallback   @   node.js:372

and

Error: the path 'style.css' does not exist in the given tree
    at Error (native)
(anonymous function)    @   C:\Program Files\Atom\resources\app.asar\node_modules\git-diff\lib\git-diff-view.js:167
tryCallOne  @   C:\Program Files\Atom\resources\app.asar\node_modules\nodegit\node_modules\promisify-node\node_modu…:37
(anonymous function)    @   C:\Program Files\Atom\resources\app.asar\node_modules\nodegit\node_modules\promisify-node\node_modu…:103
flush   @   C:\Program Files\Atom\resources\app.asar\node_modules\nodegit\node_modules\promisify-node\node_modu…:50
doNTCallback0   @   node.js:443
_tickCallback   @   node.js:372

Windows 10 x64, Atom 1.7.0-dev-579b6b7, safe mode.

/cc @joshaber

[joshaber]

Thanks @50Wliu!