Remove ShareWorktree message

Instead, create an empty worktree on guests when a worktree is first *registered*, then update it via an initial UpdateWorktree message.

This prevents the host from referencing a worktree in definition RPC responses that hasn't yet been observed by the guest. We could have waited until the entire worktree was shared, but this could take a long time, so instead we create an empty one on guests and proceed from there.

We still have randomized test failures as of this commit:

SEED=9519 MAX_PEERS=2 ITERATIONS=10000 OPERATIONS=7 ct -p zed-server test_random_collaboration

Co-Authored-By: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-Authored-By: Antonio Scandurra <me@as-cii.com>

crates/rpc/src/proto.rs

@@ -262,6 +262,7 @@ entity_messages!(
     UpdateBuffer,
     UpdateBufferFile,
     UpdateDiagnosticSummary,
+    RegisterWorktree,
     UpdateWorktree,
 );