t3600: slightly modernize style

Remove the space between redirection and file name.
Also remove unnecessary invocations of subshells, such as

	(cd submod &&
		echo X >untracked
	) &&

as there is no point of having the shell for functional purposes.
In case of a single Git command use the `-C` option to let Git cd into
the directory.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
