fix: correctly identify empty repository when commiting

The `is_empty` function exposed by libgit2 requires the default branch to be
"master". If a repository is created with a default branch not named master `is_empty`
will return false even when the repository is empty.

The fix is simply to try and access the HEAD of the repository. If the head
cant be accessed it means that the repository is still empty.

Closes #145

cog.toml

@@ -55,4 +55,5 @@ authors = [
     { signature = "Clément Poissonnier", username = "cpoissonnier" },
     { signature = "Luke Hsiao", username = "lukehsiao" },
     { signature = "Caleb Maclennan", username = "alerque" },
+    { signature = "Zshoham", username = "Zshoham" }
 ]

src/git/commit.rs

@@ -8,7 +8,7 @@ impl Repository {
         let sig = self.0.signature()?;
         let tree_id = self.0.index()?.write_tree()?;
         let tree = self.0.find_tree(tree_id)?;
-        let is_empty = self.0.is_empty()?;
+        let is_empty = self.0.head().is_err();
         let has_delta = self.get_diff(false).is_some();
 
         if !is_empty && has_delta {
@@ -62,6 +62,25 @@ mod test {
         Ok(())
     }
 
+    #[sealed_test]
+    fn first_commit_custom_branch() {
+        // Arrange
+        run_cmd! {
+            git init -b main;
+            echo changes > file;
+            git add .;
+        }
+        .expect("could not initialize git repository");
+
+        let repo = Repository::open(".").expect("could not open git repository");
+
+        // Act
+        let oid = repo.commit("feat: a test commit");
+
+        // Assert
+        assert_that!(oid).is_ok();
+    }
+
     #[sealed_test]
     fn not_create_empty_commit() -> Result<()> {
         // Arrange