Speedup by relying on filesystem ctime/mtime/filesize

On my test repo with a few text files and some images, but not too large
(194 MB in total) this speeds "add" and "commit" up as follows:

* When the command is "add", this takes 2 seconds without my ctime/mtime patch and ~0.2 without.
* When the command is "commit", this takes 2 seconds but only due to asserts that get removed for release builds..

The use-case I have in mind is that GitJournal takes 30 seconds to
startup on my phone, using this same folder of notes files. This commit
makes that usable again.

Author: lucidBrot

lib/commit.dart

@@ -128,9 +128,12 @@ extension Commit on GitRepository {
     // Write all the tree objects
     var hashMap = <String, GitHash>{};
 
+    // sort dir paths by number of slashes
     var allDirs = allTreeDirs.toList();
     allDirs.sort(dirSortFunc);
 
+    // `reversed`-> start with the deepest folders as we need the hash of
+    // all the sub-folders before we can write the parent folder.
     for (var dir in allDirs.reversed) {
       var tree = treeObjects[dir]!;
       var entries = tree.entries.unlock;
@@ -140,9 +143,10 @@ extension Commit on GitRepository {
         var leaf = entries[i];
 
         if (leaf.hash.isNotEmpty) {
-          //
-          // Making sure the leaf is a blob
-          //
+          // Making sure the leaf is a blob.
+          // This is slow because it reads every leaf,
+          // but that is alright because asserts get
+          // removed for release builds.
           assert(() {
             var leafObj = objStorage.read(leaf.hash);
             return leafObj?.formatStr() == 'blob';

lib/index.dart

@@ -36,19 +36,25 @@ extension Index on GitRepository {
       throw GitFileNotFound(filePath);
     }
 
-    // Save that file as a blob
-    var data = file.readAsBytesSync();
-    var blob = GitBlob(data, null);
-    var hash = objStorage.writeObject(blob);
-
     var pathSpec = filePath;
     if (pathSpec.startsWith(workTree)) {
       pathSpec = filePath.substring(workTree.length);
     }
-
-    // Add it to the index
+    // LB: Wait is this a linear search over all files??
+    //     Maybe... but omitting it fully does not speed things up.
     var entry = index.entries.firstWhereOrNull((e) => e.path == pathSpec);
     var stat = FileStat.statSync(filePath);
+    if (entry != null &&
+        entry.cTime.isAtSameMomentAs(stat.changed) &&
+        entry.mTime.isAtSameMomentAs(stat.modified) &&
+        entry.fileSize == stat.size) {
+      // We assume it is the same file.
+      return entry;
+    }
+
+    var data = file.readAsBytesSync();
+    var blob = GitBlob(data, null); // Hash the file (takes time!)
+    var hash = objStorage.writeObject(blob);
 
     // Existing file
     if (entry != null) {