multi_buffer: Merge adjacent matches into a single excerpt when separated by only one line (#31708)

Closes #31252

Release Notes:

- Improved displaying of project search matches or diagnostics when the
excerpts are adjacent.

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/multi_buffer/src/multi_buffer_tests.rs

@@ -1592,7 +1592,6 @@ fn test_set_excerpts_for_buffer_ordering(cx: &mut TestAppContext) {
              six
              seven
              eight
-             -----
              nine
              ten
              eleven
@@ -1848,7 +1847,6 @@ fn test_set_excerpts_for_buffer_rename(cx: &mut TestAppContext) {
         zero
         one
         two
-        -----
         three
         four
         five