tests: add write_script helper function

Many of the scripts in the test suite write small helper
shell scripts to disk. It's best if these shell scripts
start with "#!$SHELL_PATH" rather than "#!/bin/sh", because
/bin/sh on some platforms is too buggy to be used.

However, it can be cumbersome to expand $SHELL_PATH, because
the usual recipe for writing a script is:

	cat >foo.sh <<-\EOF
	#!/bin/sh
	echo my arguments are "$@"
	EOF

To expand $SHELL_PATH, you have to either interpolate the
here-doc (which would require quoting "\$@"), or split the
creation into two commands (interpolating the $SHELL_PATH
line, but not the rest of the script). Let's provide a
helper function that makes that less syntactically painful.

While we're at it, this helper can also take care of the
"chmod +x" that typically comes after the creation of such a
script, saving the caller a line.

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

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