Add File.disk_state enum to clarify filesystem states (#20776)

Motivation for this is to make things more understandable while figuring
out #20775.

This is intended to be a refactoring that does not affect behavior, but
there are a few tricky spots:

* Previously `File.mtime()` (now `File.disk_state().mtime()`) would
return last known modification time for deleted files. Looking at uses,
I believe this will not affect anything. If there are behavior changes
here I believe they would be improvements.

* `BufferEvent::DirtyChanged` is now only emitted if dirtiness actually
changed, rather than if it may have changed. This should only be an
efficiency improvement.

Release Notes:

- N/A

Co-authored-by: Mikayla Maki <mikayla@zed.dev>

crates/multi_buffer/src/multi_buffer.rs

@@ -10,9 +10,9 @@ use itertools::Itertools;
 use language::{
     language_settings::{language_settings, LanguageSettings},
     AutoindentMode, Buffer, BufferChunks, BufferRow, BufferSnapshot, Capability, CharClassifier,
-    CharKind, Chunk, CursorShape, DiagnosticEntry, File, IndentGuide, IndentSize, Language,
-    LanguageScope, OffsetRangeExt, OffsetUtf16, Outline, OutlineItem, Point, PointUtf16, Selection,
-    TextDimension, ToOffset as _, ToOffsetUtf16 as _, ToPoint as _, ToPointUtf16 as _,
+    CharKind, Chunk, CursorShape, DiagnosticEntry, DiskState, File, IndentGuide, IndentSize,
+    Language, LanguageScope, OffsetRangeExt, OffsetUtf16, Outline, OutlineItem, Point, PointUtf16,
+    Selection, TextDimension, ToOffset as _, ToOffsetUtf16 as _, ToPoint as _, ToPointUtf16 as _,
     TransactionId, Unclipped,
 };
 use smallvec::SmallVec;
@@ -2035,7 +2035,9 @@ impl MultiBuffer {
             edited |= buffer_edited;
             non_text_state_updated |= buffer_non_text_state_updated;
             is_dirty |= buffer.is_dirty();
-            has_deleted_file |= buffer.file().map_or(false, |file| file.is_deleted());
+            has_deleted_file |= buffer
+                .file()
+                .map_or(false, |file| file.disk_state() == DiskState::Deleted);
             has_conflict |= buffer.has_conflict();
         }
         if edited {