Make Diff View async in order to not lock up the UI

Selecting files often causes the UI to lock up while it loads the diff. This can take multiple seconds or even longer if the files are large and have a lot of changes (and you have increased the diff limits in settings).

Can this part of the UI be made async and run on another thread? The diff view could show a loading indicator during the process and you could continue to use SourceTree while it's loading the diff. Also would be nice to have a cancel button or cancel the process if the file selection changes.

There already seem to be a loading indicator but the process is clearly not fully async. If this can not be solved then at least provide an option to not automatically load the diff for large files when the selection changes and have a "Load diff" button in the window.

This would at least partially solve problems like SRCTREEWIN-12991