Makefile: always provide a fallback when hardlinks fail

Makefile: always provide a fallback when hardlinks fail

We make hardlinks from "git" to "git-<cmd>" built-ins and have been
careful to avoid cross-device links when linking "git-<cmd>" to
gitexecdir.

However, we were not prepared to deal with a build directory that is
incapable of making hard links within itself. This patch corrects it.

Instead of temporarily linking "git" to gitexecdir, directly link "git-
add", falling back to "cp". Try hardlinking that as "git-<cmd>", falling
back to symlinks or "cp" on error.

While at it, avoid 100+ error messages from hardlink failures when we are
going to fall back to symlinks or "cp" by redirecting the standard error
to /dev/null.

Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>