Fix LFS file not stored in LFS when uploaded/edited via API or web UI (#34367)

Files that should be stored in LFS and are uploaded/edited from the API
or web UI aren't stored in LFS. This may be a regression from #34154.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/git/attribute/checker.go

@@ -39,7 +39,12 @@ func checkAttrCommand(gitRepo *git.Repository, treeish string, filenames, attrib
 			)
 			cancel = deleteTemporaryFile
 		}
-	} // else: no treeish, assume it is a not a bare repo, read from working directory
+	} else {
+		// Read from existing index, in cases where the repo is bare and has an index,
+		// or the work tree contains unstaged changes that shouldn't affect the attribute check.
+		// It is caller's responsibility to add changed ".gitattributes" into the index if they want to respect the new changes.
+		cmd.AddArguments("--cached")
+	}
 
 	cmd.AddDynamicArguments(attributes...)
 	if len(filenames) > 0 {

modules/git/attribute/checker_test.go

@@ -57,8 +57,18 @@ func Test_Checker(t *testing.T) {
 		assert.Equal(t, expectedAttrs(), attrs["i-am-a-python.p"])
 	})
 
+	t.Run("Run git check-attr in bare repository using index", func(t *testing.T) {
+		attrs, err := CheckAttributes(t.Context(), gitRepo, "", CheckAttributeOpts{
+			Filenames:  []string{"i-am-a-python.p"},
+			Attributes: LinguistAttributes,
+		})
+		assert.NoError(t, err)
+		assert.Len(t, attrs, 1)
+		assert.Equal(t, expectedAttrs(), attrs["i-am-a-python.p"])
+	})
+
 	if !git.DefaultFeatures().SupportCheckAttrOnBare {
-		t.Skip("git version 2.40 is required to support run check-attr on bare repo")
+		t.Skip("git version 2.40 is required to support run check-attr on bare repo without using index")
 		return
 	}