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/multi_buffer/src/multi_buffer.rs

@@ -131,7 +131,6 @@ pub struct MultiBufferDiffHunk {
     pub diff_base_byte_range: Range<usize>,
     /// Whether or not this hunk also appears in the 'secondary diff'.
     pub secondary_status: DiffHunkSecondaryStatus,
-    pub secondary_diff_base_byte_range: Option<Range<usize>>,
 }
 
 impl MultiBufferDiffHunk {
@@ -3506,7 +3505,6 @@ impl MultiBufferSnapshot {
                 buffer_range: hunk.buffer_range.clone(),
                 diff_base_byte_range: hunk.diff_base_byte_range.clone(),
                 secondary_status: hunk.secondary_status,
-                secondary_diff_base_byte_range: hunk.secondary_diff_base_byte_range,
             })
         })
     }
@@ -3876,7 +3874,6 @@ impl MultiBufferSnapshot {
                         buffer_range: hunk.buffer_range.clone(),
                         diff_base_byte_range: hunk.diff_base_byte_range.clone(),
                         secondary_status: hunk.secondary_status,
-                        secondary_diff_base_byte_range: hunk.secondary_diff_base_byte_range,
                     });
                 }
             }