Port to Linux/GNUstep

[ethanc8]

Hi all, I'm currently working on a port to Linux of GitUp, using GNUstep, a FOSS implementation of AppKit and Foundation, plus a lot of GNUstep's auxiliary libraries and a few ports of some open-source Apple libraries, which you can see here: https://github.com/ethanc8/GitUp.

It requires a lot of work on GNUstep itself, which I've discussed, along with other unrelated GNUstep work, in the GNUstep discord and in various GitHub issues. If you'd like to help out, or have any questions, feel free to ask me in this issue or on GNUstep discord.

Right now, GitUpKit and the examples (GitY, GitDown, and GitDiff) build. GitY doesn't allow you to select repositories (the file picker only allows picking files), and GitDown and GitDiff allow selecting repositories and closing them but the actual diff interface is completely missing (the main window is empty). I think some of GitUpKit's unit tests fail, but I'm not sure exactly why.

A build guide is available at https://github.com/ethanc8/GitUp/blob/gnustep/GitUpKit/DebianBuildGuide.md (for Debian and Ubuntu derivatives). Outdated notes on the dependencies are available at https://github.com/ethanc8/GitUp/blob/gnustep/GitUpKit/INSTALL-GNUstep.md.

Related to #348, #160, #963

[ethanc8]

I uploaded a bit of info right here: ethanc8#1

[ethanc8]

There is a rather perplexing issue which I am facing -- that the examples GitDiff and GitDown do not display anything after selecting the repository. There are no related error messages. More details are at ethanc8#1.