Ignore parent repositories for CMake version detection

[puneetdixit200]

Summary:

• verify PROJECT_SOURCE_DIR is the Git worktree root before using git describe for CMake version detection
• keep extracted libevent source trees from adopting tags from a parent repository
• add a CTest regression that configures a fake extracted libevent tree inside a tagged parent Git repo

Tests:

• cmake -D SOURCE_DIR=$PWD -D TEST_ROOT=/tmp/libevent-version-via-git-test -P test/version-via-git-parent-repository.cmake
• cmake -S . -B /tmp/libevent-build -DCMAKE_BUILD_TYPE=Debug
• ctest --test-dir /tmp/libevent-build -R version-via-git-parent-repository --output-on-failure
• cmake --build /tmp/libevent-build -j 4
• cmake --build /tmp/libevent-build --target verify -j 4
• git diff --cached --check
• git diff --cached | /opt/homebrew/bin/bash ./checkpatch.sh -p

Fixes #1837