Keep comment spacing consistent

Comment spacing in lists is now handled consistently. Comments begin one
space character after the comma delimiter, except for the terminal element,
which has no comma and instead has two spaces before any comment.

Author: willbeason

goldens/trailing_commas.out

@@ -1,16 +1,16 @@
 Trailing comma except for last
   keep-sorted-test start
   1,
-  2, 
-  3 
+  2,
+  3
   keep-sorted-test end
 
 
 Trailing comma and a comment
 TODO: https://github.com/google/keep-sorted/issues/33 - Fix this
   keep-sorted-test start
   1,
-  2, 
+  2,
   3  # three
   keep-sorted-test end
 
@@ -19,7 +19,7 @@ TODO: https://github.com/google/keep-sorted/issues/33 - Fix this
   keep-sorted-test start
   1,
   2, # two
-  3 
+  3
   keep-sorted-test end
 
 Trailing comma and a comment, last has a comment

keepsorted/block.go

@@ -386,12 +386,20 @@ func handleTrailingComma(lgs []*lineGroup) (trimTrailingComma func([]*lineGroup)
 	}
 
 	if n := len(dataGroups); n > 1 && allEndInComma(dataGroups[0:n-1]) && !endInComma(dataGroups[n-1]) {
-		dataGroups[n-1].replaceSuffix(possibleCommentLineEnd, ", $2")
+		if dataGroups[n-1].matchesSuffix(commentLineEnd) {
+			dataGroups[n-1].replaceSuffix(commentLineEnd, ", $2")
+		} else {
+			dataGroups[n-1].append(",")
+		}
 
 		return func(lgs []*lineGroup) {
 			for i := len(lgs) - 1; i >= 0; i-- {
 				if len(lgs[i].lines) > 0 {
-					lgs[i].replaceSuffix(commaLineEnd, " $2")
+					if lgs[i].matchesSuffix(commentLineEnd) {
+						lgs[i].replaceSuffix(commaLineEnd, "  $3")
+					} else {
+						lgs[i].trimSuffix(",")
+					}
 					return
 				}
 			}
@@ -411,8 +419,8 @@ func allEndInComma(lgs []*lineGroup) bool {
 }
 
 var (
-	commaLineEnd           = regexp.MustCompile("(,)(\\s*(#|//).*)?$")
-	possibleCommentLineEnd = regexp.MustCompile("( +)?((#|//).*)?$")
+	commaLineEnd   = regexp.MustCompile("(,)( *)((#|//).*)?$")
+	commentLineEnd = regexp.MustCompile("( *)((#|//).*)$")
 )
 
 func endInComma(lg *lineGroup) bool {