feat: 新增支持通过-r参数指定差异范围生成提交信息 (#9)

- 在CLI参数中添加--range/-r选项
- 修改get_diff函数支持处理指定范围的git diff
- 更新README文档说明新功能用法

Signed-off-by: longjin <longjin@DragonOS.org>

Author: fslongjin

README.md

@@ -33,6 +33,7 @@ NOTE: All common config can be configured via `~/.fastcommit/config.toml`
    - `--branch-prefix`: prefix of the generated branch name
 - `-v, --verbosity <VERBOSITY>`: Set the detail level of the commit message. Acceptable values are `verbose` (detailed), `normal`, or `quiet` (concise). The default is `quiet`.
 - `-p, --prompt <PROMPT>`: Additional prompt to help AI understand the commit context.
+- `-r, --range <RANGE>`: Specify diff range for generating commit message (e.g. HEAD~1, abc123..def456).
 - `-h, --help`: Print help information.
 - `-V, --version`: Print version information.
 
@@ -62,6 +63,16 @@ NOTE: All common config can be configured via `~/.fastcommit/config.toml`
    fastcommit -d changes.diff -p "Fixed login page styling issues, especially button alignment"
    ```
 
+5. Generate commit message for a specific diff range:
+
+   ```bash
+   # Generate commit message for the last commit
+   fastcommit -r HEAD~1
+   
+   # Generate commit message for a range of commits
+   fastcommit -r abc123..def456
+   ```
+
 ## Contributing
 
  Contributions of code or suggestions are welcome! Please read the [Contributing Guide](CONTRIBUTING.md) first.

README_CN.md

@@ -31,6 +31,7 @@ NOTE: All common config can be configured via `~/.fastcommit/config.toml`
    - `--branch-prefix`: 生成的分支名的前缀 
 - `-v, --verbosity <VERBOSITY>`: 设置提交信息的详细级别。可选值为 `verbose`（详细）、`normal`（正常）或 `quiet`（简洁）。 默认为 `quiet`。
 - `-p, --prompt <PROMPT>`: 额外的提示信息，帮助 AI 理解提交上下文。
+- `-r, --range <RANGE>`: 指定差异范围以生成提交信息（例如：HEAD~1, abc123..def456）。
 - `-h, --help`: 打印帮助信息。
 - `-V, --version`: 打印版本信息。
 
@@ -60,6 +61,16 @@ NOTE: All common config can be configured via `~/.fastcommit/config.toml`
    fastcommit -d changes.diff -p "修复了登录页面的样式问题，特别是按钮对齐"
    ```
 
+5. 为特定的差异范围生成提交信息：
+
+   ```bash
+   # 为最近一次提交生成提交信息
+   fastcommit -r HEAD~1
+   
+   # 为指定提交范围生成提交信息
+   fastcommit -r abc123..def456
+   ```
+
 ## 贡献
 
 欢迎贡献代码或提出建议！请先阅读 [贡献指南](CONTRIBUTING.md)。

src/cli.rs

@@ -29,4 +29,11 @@ pub struct Args {
 
     #[clap(short, long, help = "Additional prompt to help AI understand the commit context")]
     pub prompt: Option<String>,
+
+    #[clap(
+        short = 'r',
+        long,
+        help = "Specify diff range (e.g. HEAD~1, abc123..def456)"
+    )]
+    pub range: Option<String>,
 }

src/generate.rs

@@ -124,11 +124,20 @@ fn extract_aicommit_message(response: &str) -> anyhow::Result<String> {
     Ok(commit_message)
 }
 
-fn get_diff(diff_file: Option<&str>) -> anyhow::Result<String> {
+fn get_diff(diff_file: Option<&str>, range: Option<&str>) -> anyhow::Result<String> {
     match diff_file {
         Some(path) => std::fs::read_to_string(path).map_err(Into::into),
         None => {
-            let output = Command::new("git").arg("diff").arg("--cached").output()?;
+            let mut cmd = Command::new("git");
+            cmd.arg("diff");
+            
+            if let Some(range_str) = range {
+                cmd.arg(range_str);
+            } else {
+                cmd.arg("--cached");
+            }
+            
+            let output = cmd.output()?;
             let diff_str = String::from_utf8_lossy(&output.stdout).into_owned();
             if diff_str.trim().is_empty() {
                 Err(anyhow::anyhow!("No changes to commit"))
@@ -140,7 +149,7 @@ fn get_diff(diff_file: Option<&str>) -> anyhow::Result<String> {
 }
 
 pub async fn generate(args: &cli::Args, config: &Config) -> anyhow::Result<String> {
-    let diff = get_diff(args.diff_file.as_deref())?;
+    let diff = get_diff(args.diff_file.as_deref(), args.range.as_deref())?;
     let message = generate_commit_message(&diff, config, args.prompt.as_deref()).await?;
     Ok(message)
 }
@@ -216,7 +225,7 @@ async fn generate_branch_name_with_ai(
 }
 
 pub async fn generate_branch(args: &cli::Args, config: &Config) -> anyhow::Result<String> {
-    let diff = get_diff(args.diff_file.as_deref())?;
+    let diff = get_diff(args.diff_file.as_deref(), args.range.as_deref())?;
     let prefix = args
         .branch_prefix
         .as_deref()