Open previous versions in editor

[altsem]

Gitu currently only looks at the workdir when opening files mentioned in a diff. Instead, get the content from git and open that instead.

Tested this out in Magit:

• Selecting the entire hunk or file takes you to the "new" file.
• Selecting an added or unchanged line takes you to the "new" file.
• Selecting a removed line takes you to the "old" file.