Allow to check out branches

The git server currently lacks branch switching capabilities, limiting both
LLMs and developers. This adds branch checkout so LLMs can
help developers add new functionality in a new feature branch.

src/git/tests/test_server.py

@@ -0,0 +1,24 @@
+import pytest
+from pathlib import Path
+import git
+from mcp_server_git.server import git_checkout
+
+def test_git_checkout_existing_branch(tmp_path: Path):
+    # Setup test repo
+    repo = git.Repo.init(tmp_path)
+    Path(tmp_path / "test.txt").write_text("test")
+    repo.index.add(["test.txt"])
+    repo.index.commit("initial commit")
+
+    # Create and test branch
+    repo.git.branch("test-branch")
+    result = git_checkout(repo, "test-branch")
+
+    assert "Switched to branch 'test-branch'" in result
+    assert repo.active_branch.name == "test-branch"
+
+def test_git_checkout_nonexistent_branch(tmp_path: Path):
+    repo = git.Repo.init(tmp_path)
+
+    with pytest.raises(git.GitCommandError):
+        git_checkout(repo, "nonexistent-branch")