Fix use of hardlinks in "make install"

The code failed to filter-out git-add properly on platforms were $X is
not empty (ATM there is only one such a platform).

Than it tried to create a hardlink to the file ($execdir/git-add) it just
removed (because git-add is first in the BUILT_INS), so ln failed (but
because stderr was redirected into /dev/null the error was never seen), and
the whole install ended up using "ln -s" instead.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1370,7 +1370,7 @@ endif
 	{ $(RM) "$$execdir/git-add$X" && \
 		ln git-add$X "$$execdir/git-add$X" 2>/dev/null || \
 		cp git-add$X "$$execdir/git-add$X"; } && \
-	{ $(foreach p,$(filter-out git-add,$(BUILT_INS)), $(RM) "$$execdir/$p" && \
+	{ $(foreach p,$(filter-out git-add$X,$(BUILT_INS)), $(RM) "$$execdir/$p" && \
 		ln "$$execdir/git-add$X" "$$execdir/$p" 2>/dev/null || \
 		ln -s "git-add$X" "$$execdir/$p" 2>/dev/null || \
 		cp "$$execdir/git-add$X" "$$execdir/$p" || exit;) } && \