Rewrite test-git-versions.sh in PHP and update the tested git versions

[lyrixx]

Summary

• Port test-git-versions.sh to test-git-versions.php, using Symfony Console and Process in the same style as tests/fixtures/generate-bundle.php, split into a build and a test command.
• Update the curated version list, picked for how long and widely each shipped as the default git on a major distro rather than a spread across history: v1.7.1 (RHEL/CentOS 6), v1.8.3.1 (RHEL/CentOS 7), v2.17.1 (Ubuntu 18.04), v2.25.1 (20.04), v2.34.1 (22.04), v2.43.0 (24.04).
• Build robustness fixes needed to actually compile these on a modern toolchain: force -std=gnu17 (GCC 15's C23 unreachable() macro collides with an identifier in git's own source), and skip imap-send/Python/Tcl-Tk extras (NO_OPENSSL, NO_PYTHON, NO_TCLTK) which aren't needed and don't build cleanly on old sources.
• Run each version's tests with an isolated HOME/XDG_CONFIG_HOME so the developer's own ~/.gitconfig (e.g. merge.conflictstyle = zdiff3, unknown to git < 2.35) can't leak into the subprocess and cause spurious failures.
• Updated the test-git-versions.sh reference in .github/CONTRIBUTING.md and gitignored /git-builds/.

Ran build then test end-to-end against all six versions locally. v2.17.1 through v2.43.0 pass cleanly. v1.7.1 and v1.8.3.1 currently fail — v1.7.1 doesn't support git config --local (added in 1.7.10) which gitlib's tests use, and both hit a DiffParser error on their diff header format. These look like genuine compatibility gaps surfaced by the tool rather than script bugs; left as-is for a separate discussion/fix.

Test plan

• php test-git-versions.php build then php test-git-versions.php test run against all six curated versions
• castor qa:cs:cs --dry-run clean on the new file