Add the ability to propose changes to a set of buffers (#18170)

This PR introduces functionality for creating *branches* of buffers that
can be used to preview and edit change sets that haven't yet been
applied to the buffers themselves.

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>
Co-authored-by: Marshall <marshall@zed.dev>

crates/multi_buffer/Cargo.toml

@@ -27,7 +27,6 @@ collections.workspace = true
 ctor.workspace = true
 env_logger.workspace = true
 futures.workspace = true
-git.workspace = true
 gpui.workspace = true
 itertools.workspace = true
 language.workspace = true