Add tests for merge conflict due to unstaged conflicting change.

[jdblischak]

@stewid I added some tests (currently failing) related to Issue #389.

I didn't update the documentation or messaging because ideally this situation can be properly handled by returning the appropriate values after a merge conflict due to unstaged changes:

• fast_forward should be returned as FALSE instead of NULL
• conflicts should be returned as TRUE instead of NULL
• sha should be returned as NA_character_ instead of NULL