tests: link shell libraries into valgrind directory

tests: link shell libraries into valgrind directory

When we run tests under valgrind, we symlink anything
executable that starts with git-* or test-* into a special
valgrind bin directory, and then make that our
GIT_EXEC_PATH.

However, shell libraries like git-sh-setup do not have the
executable bit marked, and did not get symlinked.  This
means that any test looking for shell libraries in our
exec-path would fail to find them, even though that is a
fine thing to do when testing against a regular git build
(or in a git install, for that matter).

t2300 demonstrated this problem. The fix is to symlink these
shell libraries directly into the valgrind directory.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>