syntax: fix whitespace on nested subshells

we want white space on nested subshells if its in a single line since
its ambiguous, eg we want `( (` over `((`

this shouldn't be the case for multiple lines- fix the logic that adds
white space to only do so if the two subshells are in the same line

fixes mvdan#814

cmd/shfmt/in.sh

@@ -0,0 +1,9 @@
+#!/bin/bash
+
+# OK - no blank
+( (echo "Hello world!") )
+
+# nok - blank ^ after first (
+(
+	(echo "Hello world!")
+)

syntax/printer.go

@@ -1081,6 +1081,13 @@ func (p *Printer) command(cmd Command, redirs []*Redirect) (startRedirs int) {
 		p.WriteByte('(')
 		if len(x.Stmts) > 0 && startsWithLparen(x.Stmts[0]) {
 			p.wantSpace = spaceRequired
+			if nested, ok := x.Stmts[0].Cmd.(*Subshell); ok {
+				// we only want to keep the space between two nested subshells' open brackets
+				// if its in a single line to avoid ambiguity
+				if x.Lparen.Line() != nested.Pos().Line() {
+					p.wantSpace = spaceNotRequired
+				}
+			}
 		} else {
 			p.wantSpace = spaceNotRequired
 		}

syntax/printer_test.go

@@ -616,6 +616,14 @@ var printTests = []printCase{
 		"`declare`",
 		"$(declare)",
 	},
+	{
+		"(\n(foo >redir)\n)",
+		"(\n\t(foo >redir)\n)",
+	},
+	{
+		"( (foo >redir) )",
+		"( (foo >redir))",
+	},
 }
 
 func TestPrintWeirdFormat(t *testing.T) {