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>