We need pull or fetch feature from remote

[dessasin]

Downloading changes from the remote repository is one of the git commands I use the most, but when using gitui I have to open a different console and issue the download command to download. Like the push feature, the pull command must also be present.

[extrawurst]

Pull is significant more complex and there is already an issue for this - feel free to support us with a PR :)
See #319