repository: Allow fetching via in-memory remotes

When providing remote params for FetchRemote(), then we currently create a temporary remote just to remove it after the call again. This is expensive given that removal of the remote may involve pruning all references which have been created as part of the remote's default fetch refspec. Furthermore, we also inject configuration into the fetch by calling the Ruby sidecar in order to put HTTP authorization headers into the gitconfig in order to avoid leaking credentials via the command line of git config.

Starting with git v2.31.0, we now have a better mechanism available to do this without leaking any sensitive information with the GIT_CONFIG_KEY and GIT_CONFIG_VALUE environment variables. In contrast to the command line, using environment variables avoid the issue of leaks through the command line as environment variables are by default not visible to any user but the owner.

In case this mechanism is supported, this commit thus introduces a new way of fetching remotes with an in-memory remote constructed by injecting required environment variables into the resulting process.

Part of #1773 (closed)