Introduce diff crate to unite BufferDiff and BufferChangeSet (#24392)

This is a refactoring PR that does three things:

- First, it introduces a new `diff` crate that holds the previous
contents of the `git::diff` module, plus the `BufferChangeSet` type
formerly of `project::buffer_store`. The new crate is necessary since
simply moving `BufferChangeSet` into `git::diff` results in a dependency
cycle due to the use of `language::Buffer` to represent the diff base in
`BufferChangeSet`.
- Second, it renames the two main types in the new diff crate:
`BufferDiff` becomes `BufferDiffSnapshot`, and `BufferChangeSet` becomes
`BufferDiff`. This reflects that the relationship between these two
types (immutable cheaply-cloneable "value" type + stateful "resource
type" with subscriptions) mirrors existing pairs like
`Buffer`/`BufferSnapshot`. References to "change sets" throughout the
codebase are updated to refer to "diffs" instead.
- Finally, it moves the base_text field of the new BufferDiff type to
BufferDiffSnapshot.

Release Notes:

- N/A

---------

Co-authored-by: maxbrunsfeld <max@zed.dev>

Cargo.lock

@@ -2753,6 +2753,7 @@ dependencies = [
  "ctor",
  "dashmap 6.1.0",
  "derive_more",
+ "diff 0.1.0",
  "editor",
  "env_logger 0.11.6",
  "envy",
@@ -3837,6 +3838,24 @@ dependencies = [
  "zeroize",
 ]
 
+[[package]]
+name = "diff"
+version = "0.1.0"
+dependencies = [
+ "futures 0.3.31",
+ "git2",
+ "gpui",
+ "language",
+ "log",
+ "pretty_assertions",
+ "rope",
+ "serde_json",
+ "sum_tree",
+ "text",
+ "unindent",
+ "util",
+]
+
 [[package]]
 name = "diff"
 version = "0.1.13"
@@ -4007,6 +4026,7 @@ dependencies = [
  "convert_case 0.7.1",
  "ctor",
  "db",
+ "diff 0.1.0",
  "emojis",
  "env_logger 0.11.6",
  "file_icons",
@@ -5306,6 +5326,7 @@ dependencies = [
  "anyhow",
  "collections",
  "db",
+ "diff 0.1.0",
  "editor",
  "feature_flags",
  "futures 0.3.31",
@@ -7910,9 +7931,9 @@ dependencies = [
  "clock",
  "collections",
  "ctor",
+ "diff 0.1.0",
  "env_logger 0.11.6",
  "futures 0.3.31",
- "git",
  "gpui",
  "indoc",
  "itertools 0.14.0",
@@ -9919,7 +9940,7 @@ version = "1.4.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3ae130e2f271fbc2ac3a40fb1d07180839cdbbe443c7a27e1e3c13c5cac0116d"
 dependencies = [
- "diff",
+ "diff 0.1.13",
  "yansi",
 ]
 
@@ -10015,6 +10036,7 @@ dependencies = [
  "client",
  "clock",
  "collections",
+ "diff 0.1.0",
  "env_logger 0.11.6",
  "fancy-regex 0.14.0",
  "fs",