Fix a check that prevents compiling .css files to themselves

[georgpukk]

• fixes the infinite loop issue in --watch mode (The fix for issue #806 leads to an infinity loop! #904, --watch infinite loop #853)
• previously it compared relative destination to absolute source
• p.relative(destination) is used to replace backslashes

[ReidLitkauo]

Thank you so much for this commit! I just learned SASS and this bug is the only thing preventing me from fully integrating SASS into my workflow. For now I'm just running sass from the command line every time I make an edit which is super tedious, but with this I can just set it as a daemon and forget it.

[nex3]

Thanks for the commit, and sorry it's taken so long to review! Have you been able to reproduce this in a test case? I'd like to get a regression test in place before committing a fix for this.

[georgpukk]

This is the reproducing test case (at least on Windows 10).

test("goes to infinite loop", () async {
	await d.file("test.scss", "a {b: c}").create();

	var sass = await watch(["."]);
	await expectLater(
		sass.stdout, emits('Compiled test.scss to test.css.'));
	await expectLater(sass.stdout, _watchingForChanges);
	await tickIfPoll();

	await d.file("test.scss", "r {o: g}").create();
	await expectLater(
		sass.stdout, emits('Compiled test.scss to test.css.'));

	await tickIfPoll();

	await d.file("test.scss", "x {y: z}").create();
	await expectLater(
		sass.stdout, emits('Compiled test.scss to test.css.'));

	await sass.kill();

	await d
		.file("test.css", equalsIgnoringWhitespace("x { y: z; }"))
		.validate();
});

[nex3]

Can you add that to the existing watch tests?