Add astyle-diff target in Makefile to format modified files only

[BrettDong]

Summary

None

Purpose of change

make astyle-fast still walks through all source files in the project. That could be slow on systems with slow disk I/O. It would be nice if I can just let AStyle read and format the files that I've changed.

Describe the solution

Add a astyle-diff Makefile targets that only format files modified. The list of edited files is generated by git diff --name-only.

Describe alternatives you've considered

Testing

On a clean source tree, touch a single .cpp file, and run make astyle-diff. Only that single file is read and formatted by AStyle.

Additional context

[akrieger]

make astyle already only changes files updated since the last make astyle, and it relies on make's own logic for that instead of forking out to another program. The only time that this new make command helps is running make astyle after a rebase or other git operation which changes a lot of files, and after that the timestamps are updated already so subsequent invocations only style newly changed json. Is that sufficiently painful that this is useful, in which case, should we just change make astyle to use this logic (which would strictly regress it on Windows because shelling out and git are both substantially slower on Windows than elsewhere)?