Symlink mergetools scriptlets into valgrind wrappers

Symlink mergetools scriptlets into valgrind wrappers

Since bc7a96a (mergetool--lib: Refactor tools into separate files,
2011-08-18) the mergetools and difftools related tests fail under
--valgrind because the mergetools/* scriptlets are not in the exec
path.

For now, symlink the mergetools subdir into the t/valgrind/bin
directory as a whole, since it does not contain anything of interest
to the valgrind wrappers.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>