Fix use of hardlinks in "make install"

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>