Fix limits argument of scale_{colour,fill}_manual()

[yutannihilation]

Fix #3262

At the time of #3262 (comment), I thought this should be fixed in manual_scale(). But it turns out this is a bug of ScaleDiscrete; the if block below uses limits only when pal is named:

ggplot2/R/scale-.r

Lines 778 to 783 in e6cbcc1

	if (is.null(names(pal))) {
	pal_match <- pal[match(as.character(x), limits)]
	} else {
	pal_match <- pal[match(as.character(x), names(pal))]
	pal_match <- unname(pal_match)
	}

I see no reason to ignore limits here. (This code has been here since Scale was first implemented (1877b6d), so there might have been some limitation?) This PR fixes the issue by limiting the pal first before matching it against the supplied values.

[thomasp85]

LGTM - if everything checks out after resolving conflicts you are free to merge

[yutannihilation]

Thanks