git: git_show prints Python reprs instead of git's commit format #4998

Problem

git_show's header is built from Python objects, so it prints a quoted sha, a <git.Actor …> repr and a datetime repr that includes a memory address. For an added file it prints --- None where git prints --- /dev/null.

Where it is pinned

src/git/tests/test_protocol.py: test_git_show_commit_with_parent, test_git_show_initial_commit_diffs_against_empty_tree, test_git_show_rename_only_commit_has_header_and_no_patch, test_git_show_binary_commit, added in #4974 (Wave 1 of #4857). Each of these tests carries a KNOWN BUG marker: it asserts the current, wrong behavior, so the fix has to change it.

Expected

The header uses git's own format (sha, Author: Name <email>, an ISO date), and new or deleted files use /dev/null.

Done when

• The behavior above is fixed, in the legacy-era (2025-11-25) server on SDK 1.x.
• The pinning tests assert the correct behavior, and their KNOWN BUG markers are gone.
• The per-file 90% coverage gate still passes.