repository: Disable misleading dangling checks in git-fsck(1)

The Fsck RPC uses git-fsck(1) to inform the user about repository corruption. In case the Git command returns with a non-zero exit code the RPC will return both its stdout and stderr to the caller via the Error field that is part of the FsckResponse.

By default, git-fsck(1) will not only print information about repository corruption though, but also about dangling objects that aren't reachable via any reference. But as it is totally expected that repositories have dangling objects, it is only distracting from the actual error messages that the repository owner would care about.

Stop git-fsck(1) from checking for dangling objects to improve the signal to noise ratio. While at it, let's also explicitly ask it to not print any progress -- while it wouldn't do that anyway because it is not printing to a TTY, it stops the next reader of the code from wondering whether it would ever do that.

Closes gitlab#376765 (closed).