Add an option to leave a conflicted file unstaged after conflict resolution

Currently the dialog offers two choices after the resolution is done:

- Stage: Mark the conflict as resolved and stage the file

- Cancel: Revert the file into conflicted state.

It would be nice to have a third option that will allow the user to mark the conflict as resolved and the file unstaged. I believe the Git command in this case is simply "git reset HEAD <file>".