go/ast: remove many blank lines in SortImports

Currently only one blank line is checked to be removed.
Changing sort.Sort may lead to more blank lines.
Let's remove them all.

It fixes a bug found by https://golang.org/cl/15688

Change-Id: I682cc23ecd7b10d9b6feb160da040a155297f578
Reviewed-on: https://go-review.googlesource.com/17440
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/go/ast/import.go

@@ -43,8 +43,10 @@ func SortImports(fset *token.FileSet, f *File) {
 		if len(d.Specs) > 0 {
 			lastSpec := d.Specs[len(d.Specs)-1]
 			lastLine := fset.Position(lastSpec.Pos()).Line
-			if rParenLine := fset.Position(d.Rparen).Line; rParenLine > lastLine+1 {
-				fset.File(d.Rparen).MergeLine(rParenLine - 1)
+			rParenLine := fset.Position(d.Rparen).Line
+			for rParenLine > lastLine+1 {
+				rParenLine--
+				fset.File(d.Rparen).MergeLine(rParenLine)
 			}
 		}
 	}