Implement staging of partially-staged hunks (#25520)

Closes: #25475 

This PR makes it possible to stage uncommitted hunks that overlap but do
not coincide with an unstaged hunk.

Release Notes:

- Made it possible to stage hunks that are already partially staged

---------

Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Max <max@zed.dev>

crates/text/src/text.rs

@@ -2934,6 +2934,7 @@ impl ToOffset for Point {
 }
 
 impl ToOffset for usize {
+    #[track_caller]
     fn to_offset(&self, snapshot: &BufferSnapshot) -> usize {
         assert!(
             *self <= snapshot.len(),