Makefile: remove and create libgit.a from scratch.

Makefile: remove and create libgit.a from scratch.

Foolishly I renamed diff.o around which caused an old diff.o
taken out of libgit.a and got linked into resulting binary and
exhibited mysterious breakage for many people.  This borrows
from the kernel Makefile (scripts/Makefile.build) to first remove
the target and then recreate.

Signed-off-by: Junio C Hamano <junkio@cox.net>