[WIP] Add FilePatch-header

[simurai]

This adds a header to the FilePatch view. It could be used to add some more infos or buttons.

• For example when talking to @kuychaco about File diff selection improvements and add stage/unstage context menu option #478 the idea came up to add a checkbox (toggle) so you could see what mode you're in.
• Have a button to switch to either the staged or unstaged changes of a file.
• Have a button to open the file.
• etc.

Concerns

Overkill? For example the toggle might makes it feel more complicated than it should be. Having to pay attention to what mode you're in. Switching between staged/unstaged changes without having to find it in the list sounds useful though.

[simurai]

Ok, even without trying, I think the toggle is a bad idea. Because seeing the toggle switch when clicking on hunk headers or lines probably feels pretty irritating.

Also, I forgot that the file name is shown in the status-bar, in case it's long and you can't read it in the tab.

@kuychaco So not sure. We can also close this again and come back to it if we really think it's necessary.

[kuychaco]

Hmm those are all good points @simurai. I do like the open file and view staged/unstaged version affordances. We could also add buttons for stage/unstage all, discard all, and undo discard. What do you think about keeping the bar just for these?

[kuychaco]

Okay, I pulled the file diff header into #487, took out the toggle, wired up everything and added a Undo Last Discard button.

I'll go ahead and close this for now and we can revisit the hunk/line mode indication at a later point if we feel the need to.