syntax: allow escaped newlines to precede unquoted words again

As part of fixing #832, to prevent breaking some programs with escaped
newlines, I went slightly too far with the tightening of the rules.
The input script

	foo <<< \
		"bar baz"

would now reformat as

	foo <<< "\
	bar baz"

The script is functionally the same, but it looks objectively worse.
It is harmless to allow a leading escaped newline when not quoted,
as it doesn't break any programs nor result in worse formatting.

Note that the script

	foo=\
	bar

will still reformat as `foo=bar`, so in that case we keep the recent
change in behavior. That case is different, as we can't safely indent
the continuing line - thus we can't produce good formatting unless we
join the lines.

While here, remove incomplete code that I had left commented out in the
last patch, and then forgot to remove before merging.

Fixes #873.

syntax/printer.go

@@ -593,6 +593,14 @@ func (p *Printer) comments(comments ...Comment) {
 }
 
 func (p *Printer) wordParts(wps []WordPart, quoted bool) {
+	// We disallow unquoted escaped newlines between word parts below.
+	// However, we want to allow a leading escaped newline for cases such as:
+	//
+	//   foo <<< \
+	//     "bar baz"
+	if !quoted && !p.singleLine && wps[0].Pos().Line() > p.line {
+		p.bslashNewl()
+	}
 	for i, wp := range wps {
 		var next WordPart
 		if i+1 < len(wps) {
@@ -1413,10 +1421,7 @@ func (p *Printer) assigns(assigns []*Assign) {
 			// because that can result in indentation, thus
 			// splitting "foo=bar" into "foo= bar".
 			p.line = a.Value.Pos().Line()
-			// Similar to the above, we want to print the word as if it were
-			// quoted, as otherwise escaped newlines could split
 			p.word(a.Value)
-			// p.wordParts(a.Value.Parts, true)
 		} else if a.Array != nil {
 			p.wantSpace = spaceNotRequired
 			p.WriteByte('(')

syntax/printer_test.go

@@ -144,14 +144,16 @@ var printTests = []printCase{
 		"a bb\\\ncc d",
 		"a bbcc d",
 	},
-	samePrint("a \\\n\tb \\\n\tc \\\n\t;"),
-	samePrint("a=1 \\\n\tb=2 \\\n\tc=3 \\\n\t;"),
+	samePrint("a \\\n\tb \\\n\t\"c\" \\\n\t;"),
+	samePrint("a=1 \\\n\tb=2 \\\n\tc=\"3\" \\\n\t;"),
 	{
 		"a=\\\nfoo\nb=\\\n\"bar\"\nc=\\\n'baz'",
 		"a=foo\nb=\"bar\"\nc='baz'",
 	},
 
 	samePrint("if a \\\n\t; then b; fi"),
+	samePrint("a > \\\n\tfoo"),
+	samePrint("a <<< \\\n\t\"foo\""),
 	samePrint("a 'b\nb' c"),
 	samePrint("a $'b\nb' c"),
 	{