scm: unable to discard changes for a newly created file

[vince-fugnitto]

Bug Description:

There is a bug when attempting to discard changes in the scm-view for a file that is newly created.

command.ts:340 Uncaught (in promise) Error: The command 'file.delete' cannot be executed. There are no active handlers available for the command.
    at CommandRegistry.executeCommand (command.ts:340:29)
    at discardSingle (git-scm-provider.ts:401:45)
    at git-scm-provider.ts:404:24
    at Array.map (<anonymous>)
    at GitScmProvider.discard (git-scm-provider.ts:392:19)
    at async ProgressService.withProgress (progress-service.ts:77:20)
    at async CommandRegistry.executeCommand (command.ts:336:28)

Steps to Reproduce:

1. start the application with a workspace under version control (ex: theia)
2. create a new file (ex: touch a.txt)
3. open the scm-view and attempt to discard the file using the discard changes toolbar item