实现Index析构自动保存：Drop for Index

src/commands/add.rs

@@ -42,8 +42,6 @@ pub fn add(files: Vec<String>, all: bool, mut update: bool) {
     for file in &files {
         add_a_file(file, &mut index);
     }
-
-    index.save();
 }
 
 fn add_a_file(file: &Path, index: &mut Index) {

src/commands/status.rs

@@ -65,6 +65,10 @@ pub fn changes_to_be_committed() -> Changes {
     change
 }
 
+pub fn changes_to_be_staged() {
+    unimplemented!()
+}
+
 /** 分为两个部分
 1. unstaged: 暂存区与工作区比较
 2. staged to be committed: 暂存区与HEAD(最后一次Commit::Tree)比较，即上次的暂存区