[git] Error message when discarding all changes

[kittaakos]

Bug Description:

Steps to Reproduce:

1. Change a file,
2. Select Discard All Changes, confirm the dialog.
3. See the error message.

I found this when I was reviewing #9324.

Error from the backend:

root ERROR The command `git checkout-index -f -u HEAD -- CHANGELOG.md` exited with an unexpected code: 1. The caller should either handle this error, or expect that exit code.
root ERROR git checkout-index: HEAD is not in the cache

root ERROR Request unstage failed with error: git checkout-index: HEAD is not in the cache
 GitError: git checkout-index: HEAD is not in the cache

    at new GitError (/workspace/theia/node_modules/dugite-extra/lib/core/git.js:87:28)
    at Object.<anonymous> (/workspace/theia/node_modules/dugite-extra/lib/core/git.js:262:27)
    at step (/workspace/theia/node_modules/dugite-extra/lib/core/git.js:56:23)
    at Object.next (/workspace/theia/node_modules/dugite-extra/lib/core/git.js:37:53)
    at fulfilled (/workspace/theia/node_modules/dugite-extra/lib/core/git.js:28:58)
    at processTicksAndRejections (internal/process/task_queues.js:94:5)

Additional Information

• Operating System:
• Theia Version:

[FinnChen]

I submitted a PR to fix this issue. Anyone can help to review the PR? eclipse-theia/dugite-extra#57