Implement Git diff visualization for individual commits

[jonasehrlich]

Implement a visualization of diffs of individual commits in the Git Panel (#5).

It should both offer to display the diff of each commit in some way, as well as the total diff of the tree. For rendering react-diff-view or react-diff-viewer-continued could be used. The rendering of the diff could be done in a separate dialog.

Visualization of the diffs requires #4

[jonasehrlich]

Closed in favor of #82