Merge branch 'ab/make-cleanup'

Reorganize Makefile to allow building git.o and other essential objects without extra stuff needed only for testing. * ab/make-cleanup: Makefile: add {program,xdiff,test,git,fuzz}-objs & objects targets Makefile: split OBJECTS into OBJECTS and GIT_OBJS Makefile: sort OBJECTS assignment for subsequent change Makefile: split up long OBJECTS line Makefile: guard against TEST_OBJS in the environment