Open File shortcut does not work if tree is clean

[calvn]

Issue Type: Bug

"Open File" shortcut/command does not work on an opened diff-view file if the working tree becomes clean (e.g. after a commit). Oddly, pressing the button near the top right corner for this action does work.

"Open Changes" in a file doesn't work on a clean tree which is expected, but going from a diff-view back to the file after changes have been committed should be a valid flow IMO.

It'd be nice to have a shortcut by default to toggle between the two.

Repro steps:

1. Have a dirty working tree
2. Go to the modified file via "Open Changes" button or shortcut
3. Keep that diff-view file open
4. Git commit the changes
5. Attempt to "Open File" via the command pallette (shift+cmd P) -> "Git: Open File", nothing happens
6. Click on the "Open File" button near top right corner. This takes me to the corresponding file.

VS Code version: Code 1.28.0 (431ef9d, 2018-10-04T16:40:40.180Z)
OS version: Darwin x64 18.0.0