Add fs::MTime newtype to encourage != instead of > (#20830)

See ["mtime comparison considered
harmful"](https://apenwarr.ca/log/20181113) for details of why
comparators other than equality/inequality should not be used with
mtime.

Release Notes:

- N/A

crates/semantic_index/src/embedding_index.rs

@@ -7,6 +7,7 @@ use anyhow::{anyhow, Context as _, Result};
 use collections::Bound;
 use feature_flags::FeatureFlagAppExt;
 use fs::Fs;
+use fs::MTime;
 use futures::stream::StreamExt;
 use futures_batch::ChunksTimeoutStreamExt;
 use gpui::{AppContext, Model, Task};
@@ -17,14 +18,7 @@ use project::{Entry, UpdatedEntriesSet, Worktree};
 use serde::{Deserialize, Serialize};
 use smol::channel;
 use smol::future::FutureExt;
-use std::{
-    cmp::Ordering,
-    future::Future,
-    iter,
-    path::Path,
-    sync::Arc,
-    time::{Duration, SystemTime},
-};
+use std::{cmp::Ordering, future::Future, iter, path::Path, sync::Arc, time::Duration};
 use util::ResultExt;
 use worktree::Snapshot;
 
@@ -451,7 +445,7 @@ struct ChunkFiles {
 
 pub struct ChunkedFile {
     pub path: Arc<Path>,
-    pub mtime: Option<SystemTime>,
+    pub mtime: Option<MTime>,
     pub handle: IndexingEntryHandle,
     pub text: String,
     pub chunks: Vec<Chunk>,
@@ -465,7 +459,7 @@ pub struct EmbedFiles {
 #[derive(Debug, Serialize, Deserialize)]
 pub struct EmbeddedFile {
     pub path: Arc<Path>,
-    pub mtime: Option<SystemTime>,
+    pub mtime: Option<MTime>,
     pub chunks: Vec<EmbeddedChunk>,
 }