Use Git identity for command-line commits

[Gabriel-Darbord]

Summary

• Stop forcing compatibility author environment variables around command-line Git commits
• Let Git resolve repository/global commit identity normally
• Populate the synthetic Iceberg commit author from the actual Git commit
• Add a regression test proving repo-local Git identity is used

Verification

• Pharo 13: PharoCompatibilityP12SurfacePharo13Test passed, 4 tests
• Pharo 13: PharoCompatibility test packages passed, 9 tests
• Committed this branch through repository_commit and verified the actual Git author is Gabriel-Darbord <gabriel.darbord@berger-levrault.com>