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

diff --cc Makefile
Simple merge