go/printer: set prefix correctly when all comment lines blank

Fixes (second half of?) golang issue 5128.

In stripCommonPrefix, the prefix was correctly calculated in all cases
except when there are more than 2 lines but all lines except the first
and last are blank.
This fix detects that case and correctly sets the prefix calculated
from the last line.
That fixes the issue where gofmt was continuously adding indentation in
such cases on every format operation.

101/printer/printer.go

@@ -506,17 +506,24 @@ func stripCommonPrefix(lines []string) {
 	var prefix string
 	if len(lines) > 2 {
 		first := true
+		allBlank := true
 		for i, line := range lines[1 : len(lines)-1] {
 			switch {
 			case isBlank(line):
 				lines[1+i] = "" // range starts with lines[1]
 			case first:
 				prefix = commonPrefix(line, line)
 				first = false
+				allBlank = false
 			default:
 				prefix = commonPrefix(prefix, line)
+				allBlank = false
 			}
 		}
+		if allBlank { // all lines other than first and last are blank
+			line := lines[len(lines)-1]
+			prefix = commonPrefix(line, line)
+		}
 	} else { // len(lines) == 2, lines cannot be blank (contain /* and */)
 		line := lines[1]
 		prefix = commonPrefix(line, line)