Support git show object specs

[oaslananka]

Summary

• Support Git object specs such as HEAD:path/to/file in git_show.
• Preserve the existing formatted commit output for normal commit revisions.
• Add a regression test for showing a nested blob through HEAD:logic/position_realtime_infos.py.

Fixes #1682

Tests

• python -m pytest tests/test_server.py::test_git_show_blob_object_spec tests/test_server.py::test_git_show tests/test_server.py::test_git_show_initial_commit tests/test_server.py::test_git_show_rejects_flag_injection tests/test_server.py::test_git_show_rejects_malicious_refs -q
• python -m pytest tests/test_server.py -q
• python -m py_compile src/mcp_server_git/server.py
• git diff --check