[Feature Request] Allow editing text, and undo + redo in Diff View

[sken130]

As titled. Currently when viewing and editing diffs, I have to resort to TortoiseGit.

I think this item can be merged with #2168

[love-linger]

As I said in #2458, SourceGit shows the output of git diff command, instead of reading the old and new revision of file then calculate the diff. And the most important thing is SourceGit support the combined/stacked diff view - that means the old side and the new side both shares some unchanged lines.

Imagine that every time you type a character, SourceGit has to re-run git diff to recalculate new differences (and it's possible that after a certain input, there are no differences between the file and its original version). It also needs to compute the original cursor position and restore it in the new results. What's more, git diff can sometimes take quite a long time to execute...

I think this is also why the diff views of most Git GUI tools do not support editing. In contrast, the vast majority of diff tools (such as TortoiseMerge you mentioned) allow editing because their two panes display the two original files themselves...