Add DisconnectGitAlternates RPC

We need these to recover from inconsistent state on disk.

Edit: I rediscovered ReduplicateRepository. I think I still want to make it faster.

Edit 2: No, it is better not to radically change behavior of existing RPC's. I'm adding DisconnectGitAlternates as a new RPC after all.

Edited Apr 10, 2019 by Jacob Vosmaer

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information