feat: Add git branch functionality and unit tests

This commit introduces the `git branch` tool to the MCP Git server, allowing users to list branches with various filtering options.

Changes include:
- Implemented `git_branch` function in `src/git/src/mcp_server_git/server.py` to support listing local, remote, and all branches, as well as filtering by `contains` and `not_contains` SHA values.
- Added comprehensive unit tests for the `git branch` functionality in `src/git/tests/test_server.py`, covering different branch types and commit filtering scenarios.
- Updated `src/git/README.md`.

src/git/tests/test_server.py

@@ -1,7 +1,7 @@
 import pytest
 from pathlib import Path
 import git
-from mcp_server_git.server import git_checkout
+from mcp_server_git.server import git_checkout, git_branch
 import shutil
 
 @pytest.fixture
@@ -27,4 +27,44 @@ def test_git_checkout_existing_branch(test_repository):
 def test_git_checkout_nonexistent_branch(test_repository):
 
     with pytest.raises(git.GitCommandError):
-        git_checkout(test_repository, "nonexistent-branch")
+        git_checkout(test_repository, "nonexistent-branch")
+
+def test_git_branch_local(test_repository):
+    test_repository.git.branch("new-branch-local")
+    result = git_branch(test_repository, "local")
+    assert "new-branch-local" in result
+
+def test_git_branch_remote(test_repository):
+    # GitPython does not easily support creating remote branches without a remote.
+    # This test will check the behavior when 'remote' is specified without actual remotes.
+    result = git_branch(test_repository, "remote")
+    assert "" == result.strip()  # Should be empty if no remote branches
+
+def test_git_branch_all(test_repository):
+    test_repository.git.branch("new-branch-all")
+    result = git_branch(test_repository, "all")
+    assert "new-branch-all" in result
+
+def test_git_branch_contains(test_repository):
+    # Create a new branch and commit to it
+    test_repository.git.checkout("-b", "feature-branch")
+    Path(test_repository.working_dir / Path("feature.txt")).write_text("feature content")
+    test_repository.index.add(["feature.txt"])
+    commit = test_repository.index.commit("feature commit")
+    test_repository.git.checkout("master")
+
+    result = git_branch(test_repository, "local", contains=commit.hexsha)
+    assert "feature-branch" in result
+    assert "master" not in result
+
+def test_git_branch_not_contains(test_repository):
+    # Create a new branch and commit to it
+    test_repository.git.checkout("-b", "another-feature-branch")
+    Path(test_repository.working_dir / Path("another_feature.txt")).write_text("another feature content")
+    test_repository.index.add(["another_feature.txt"])
+    commit = test_repository.index.commit("another feature commit")
+    test_repository.git.checkout("master")
+
+    result = git_branch(test_repository, "local", not_contains=commit.hexsha)
+    assert "another-feature-branch" not in result
+    assert "master" in result