Diff mode doesn't work as expected with existing workspace

[kevingunn-wk]

Related to https://github.com/xolox/vim-session/issues/171:

When trying to run vim in diff mode, the diff files open as tabs and not in a split pane as expected.

Reproduce:

1. Open a file and allow workspace to be created
2. Close vim
3. Open in diff mode: vim -d a.txt b.txt
4. Note two tabs are added to the existing session/workspace.

Workaround:

1. Before using vim diff, delete Session.vim
2. Open in diff mode and compare files as expected

Is it possible to disable workspaces when vim is opened in diff mode? Don't restore a session or create a session from diff mode? Other option would be to open the files as expected in their own tab and add the to the workspace session.