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>