docs: enhance docstrings and add test coverage

- Update docstrings in __main__.py and prompt.py to clarify usage of 'previous_commit_message' parameter
- Add new test file test_prompt.py to cover basic, template, previous message, and instruction scenarios

These changes improve the clarity of the documentation and ensure that the commit message generation process is thoroughly tested.

gcop/__main__.py

@@ -65,8 +65,9 @@ def generate_commit_message(
     Args:
         diff(str): git diff
         instruction(Optional[str]): additional instruction. Defaults to None.
-        previous_commit_message(Optional[str]): previous commit message. Defaults to
-            None.
+        previous_commit_message(Optional[str]): previous commit message. At the first
+            time, it's usually empty. It always uses when you are improving the
+            commit message or providing feedback. Defaults to None.
 
     Returns:
         str: git commit message with ai generated.

gcop/prompt.py

@@ -91,8 +91,9 @@ def get_commit_instrcution(
         diff (str): git diff
         commit_template (Optional[str], optional): commit template. Defaults to None.
         instruction (Optional[str], optional): additional instruction. Defaults to None.
-        previous_commit_message (Optional[str], optional): previous commit message.
-            Defaults to None.
+        previous_commit_message (Optional[str], optional): previous commit message. At
+            the first time, it's usually empty. It always uses when you are
+            improving the commit message or providing feedback.
 
     Returns:
         str: system prompt for generating commit messages

tests/test_prompt.py

@@ -0,0 +1,66 @@
+import pytest
+
+from gcop.prompt import get_commit_instrcution
+
+
+def test_get_commit_instruction_basic():
+    """Test basic commit instruction generation."""
+    diff = "test diff content"
+    result = get_commit_instrcution(diff)
+    assert isinstance(result, str)
+    assert "test diff content" in result
+
+
+def test_get_commit_instruction_with_template():
+    """Test commit instruction with custom template."""
+    diff = "test diff"
+    template = """
+    <example>
+    feat: custom template
+    </example>
+    """
+    result = get_commit_instrcution(diff, commit_template=template)
+    assert isinstance(result, str)
+    assert "custom template" in result
+    assert "test diff" in result
+
+
+def test_get_commit_instruction_with_previous():
+    """Test commit instruction with previous commit message."""
+    diff = "test diff"
+    prev_msg = "fix: previous commit"
+    result = get_commit_instrcution(diff, previous_commit_message=prev_msg)
+    assert isinstance(result, str)
+    assert "previous commit" in result
+    assert "test diff" in result
+
+
+def test_get_commit_instruction_with_instruction():
+    """Test commit instruction with additional instruction."""
+    diff = "test diff"
+    instruction = "Please make it more detailed"
+    result = get_commit_instrcution(diff, instruction=instruction)
+    assert isinstance(result, str)
+    assert "Please make it more detailed" in result
+    assert "test diff" in result
+
+
+def test_get_commit_instruction_all_params():
+    """Test commit instruction with all parameters."""
+    diff = "test diff"
+    template = "<example>template</example>"
+    prev_msg = "previous message"
+    instruction = "make it better"
+
+    result = get_commit_instrcution(
+        diff,
+        commit_template=template,
+        previous_commit_message=prev_msg,
+        instruction=instruction,
+    )
+
+    assert isinstance(result, str)
+    assert "test diff" in result
+    assert "template" in result
+    assert "previous message" in result
+    assert "make it better" in result