Decouple core Yaak logic from Tauri (#354)

crates/yaak-git/src/add.rs

@@ -0,0 +1,16 @@
+use crate::error::Result;
+use crate::repository::open_repo;
+use git2::IndexAddOption;
+use log::info;
+use std::path::Path;
+
+pub fn git_add(dir: &Path, rela_path: &Path) -> Result<()> {
+    let repo = open_repo(dir)?;
+    let mut index = repo.index()?;
+
+    info!("Staging file {rela_path:?} to {dir:?}");
+    index.add_all(&[rela_path], IndexAddOption::DEFAULT, None)?;
+    index.write()?;
+
+    Ok(())
+}