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

t/test-lib.sh

@@ -884,8 +884,13 @@ then
 	}
 
 	make_valgrind_symlink () {
-		# handle only executables
-		test -x "$1" || return
+		# handle only executables, unless they are shell libraries that
+		# need to be in the exec-path.  We will just use "#!" as a
+		# guess for a shell-script, since we have no idea what the user
+		# may have configured as the shell path.
+		test -x "$1" ||
+		test "#!" = "$(head -c 2 <"$1")" ||
+		return;
 
 		base=$(basename "$1")
 		symlink_target=$GIT_BUILD_DIR/$base