Run clang-format only on C++ files changed in branch

[tenpercent]

Summary

This PR optimizes the Jenkins CI clang-format check to only run on C++ files that have been modified in the current branch, rather than checking all files in the repository.

Changes

• Detect merge base with origin/develop branch using git merge-base
• Check only C++ files modified since merge base: *.h, *.hpp, *.cpp, *.h.in, *.hpp.in, *.cpp.in, *.cl
• Exclude build/ and include/rapidjson directories (same as before)
• Provide clear error messages when formatting issues are found
• Exit cleanly when no C++ files are changed
• Preserve original logic:
  • When RUN_CPPCHECK=false: Run clang-format only (on changed files)
  • When RUN_CPPCHECK=true: Run both clang-format (on changed files) and cppcheck

Benefits

• Faster CI times for branches that modify only a few files
• Same coverage - all changed C++ files are still checked
• Better feedback - shows exactly which files are being checked and which have issues
• Maintains standards - code formatting requirements unchanged

Test Plan

• Verify clang-format runs on branches with C++ file changes
• Verify graceful exit on branches without C++ file changes
• Verify both RUN_CPPCHECK=true and RUN_CPPCHECK=false scenarios work correctly
• Confirm formatting errors are properly detected and reported