treat err assignments as a simple error check

We considered the following pattern a "simple" error check,
where err was declared with `:=` and immediately followed by an if:

    x, err := f()

    if err != nil {...}

We would then remove the empty line before the error check.
However, we wouldn't do the same if x and err were already declared:

    x, err = f()

    if err != nil {...}

The second form is still relatively common, and there's no good reason
for the check to treat the two cases differently.

Fixes #271.

format/format.go

@@ -866,9 +866,9 @@ func (f *fumpter) stmts(list []ast.Stmt) {
 			continue // not an if following another statement
 		}
 		as, ok := list[i-1].(*ast.AssignStmt)
-		if !ok || as.Tok != token.DEFINE ||
+		if !ok || (as.Tok != token.DEFINE && as.Tok != token.ASSIGN) ||
 			!identEqual(as.Lhs[len(as.Lhs)-1], "err") {
-			continue // not "..., err := ..."
+			continue // not ", err :=" nor ", err ="
 		}
 		be, ok := ifs.Cond.(*ast.BinaryExpr)
 		if !ok || ifs.Init != nil || ifs.Else != nil {

testdata/script/newline-errcheck.txtar

@@ -53,6 +53,12 @@ func f() {
 			panic(err)
 		}
 	}
+
+	n5, err = Do2()
+
+	if err != nil {
+		panic(err)
+	}
 }
 -- foo.go.golden --
 package p
@@ -101,4 +107,9 @@ func f() {
 			panic(err)
 		}
 	}
+
+	n5, err = Do2()
+	if err != nil {
+		panic(err)
+	}
 }