Cannot fetch unless a remote is defined

Hi,

Thank you for pygit2! I'm using it extensively for git tooling in Ubuntu, but occasionally I find some corner where a feature doesn't exist and I have to resort to calling a git subprocess.

The git CLI allows me to use git fetch without having a remote defined. I can just specify a URL. I can't find any documentation on this in pygit2 though. It looks like I have to define a remote, with a name, first. Since I don't necessarily own the repository, this requires an ugly workaround because I have to be concerned about name collisions, crashing without having removed a temporary remote, etc.

Any chance of adding API functionality to fetch (and presumably) push against remotes with just a URL and no mandatory repository configuration please?

首先检查 pygit2 现有的远程定义以及 fetch/push 入口，同时对照 issue 中描述的 git CLI 行为。确定如何在不创建具名远程的情况下仅使用 URL 执行 fetch 和 push，然后通过针对性测试验证该行为；当这两个操作都能接受 URL 而无需仓库配置时，即视为完成。