revision.o is included in libgit.a which is in $(GITLIBS), so we don't
need to include is separately.  This fixes compilation with
"-fwhole-program" which otherwise fails with messages like this:

  libgit.a(revision.o): In function `mark_tree_uninteresting':
  /home/john/src/git/revision.c:108: multiple definition of `mark_tree_uninteresting'
  /tmp/ccKQRkZV.ltrans2.ltrans.o:/home/john/src/git/revision.c:108: first defined here
Signed-off-by: NJohn Keeping <john@keeping.me.uk>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>