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>

t/test-lib.sh

@@ -381,11 +381,20 @@ test_config () {
 	git config "$@"
 }
 
+
 test_config_global () {
 	test_when_finished "test_unconfig --global '$1'" &&
 	git config --global "$@"
 }
 
+write_script () {
+	{
+		echo "#!${2-"$SHELL_PATH"}" &&
+		cat
+	} >"$1" &&
+	chmod +x "$1"
+}
+
 # Use test_set_prereq to tell that a particular prerequisite is available.
 # The prerequisite can later be checked for in two ways:
 #