Preview git status updates

Preview only changes since file renamed (#263)

Previously, the git diff preview for a renamed path shows the entire file as being added. A more useful diff is to view only the modifications made to the file content. To accomplish this, we need a special condition for renames to diff the current (post-rename) path with the original path.

In addition, this PR fixes git status previews for renamed paths, which was accidentally broken in 38896f2.

Cover all possible unmerged statuses (309975c)

In #262, my research showed that only 4 of the 7 unmerged statuses listed in git status' help were
possible. However, upon experimentation, I realized all 7 are possible. The three I left out are easily reproducible by merging two branches
that renamed the same file but to different names. In this commit, I add them back in.