Allow using git reference for branch/tag/commit

src/cmd/shelf.rs

@@ -58,6 +58,7 @@ pub fn execute(_args: &ArgMatches) -> Result<()> {
     let shelf: Shelf = toml::from_str(&contents)?;
 
     let _ = std::fs::remove_dir_all("shelf");
+    let _ = std::fs::remove_dir_all("repositories");
     let _ = MDBook::init("shelf").create_gitignore(false).build();
 
     let index_file_name = "shelf/src/index.md";
@@ -86,14 +87,39 @@ pub fn execute(_args: &ArgMatches) -> Result<()> {
                 checkout_path.clone()
             };
 
-            let _repo = match git2::Repository::open(&checkout_path) {
+            let repo = match git2::Repository::open(&checkout_path) {
                 Ok(repo) => repo,
                 Err(_) => match git2::Repository::clone(&url, &checkout_path) {
                     Ok(repo) => repo,
                     Err(e) => panic!("failed to clone: {}", e),
                 },
             };
 
+            if let Some(refname) = &sb.git_ref {
+                // let refname = "master"; // or a tag (v0.1.1) or a commit (8e8128)
+                let (object, reference) =
+                    if let Ok((object, reference)) = repo.revparse_ext(refname) {
+                        (object, reference)
+                    } else if let Ok((object, reference)) =
+                        repo.revparse_ext(&format!("origin/{refname}"))
+                    {
+                        (object, reference)
+                    } else {
+                        panic!("Could not checkout {refname}");
+                    };
+
+                repo.checkout_tree(&object, None)
+                    .expect("Failed to checkout");
+
+                match reference {
+                    // gref is an actual reference like branches or tags
+                    Some(gref) => repo.set_head(gref.name().unwrap()),
+                    // this is a commit, not a reference
+                    None => repo.set_head_detached(object.id()),
+                }
+                .expect("Failed to set HEAD");
+            }
+
             process_book(&book_path.to_str().unwrap(), &mut index_file, &mut summary)?
         } else if let Some(path) = &sb.path {
             process_book(path, &mut index_file, &mut summary)?

src/config.rs

@@ -646,6 +646,9 @@ pub struct ShelfBook {
     pub path: Option<String>,
     /// git url
     pub git_url: Option<String>,
+    /// reference to checkout in git
+    /// This can be a branch, commit or tag
+    pub git_ref: Option<String>,
 }
 
 #[derive(Deserialize, Debug)]