FIX: Single semicolon at beginning was indented.

The workaround here is to add a double semicolon, iff the start comment is a
single semicolon.  This also required some fixing of the whitespace
calculations.  This commit is for issue
387 (AndreaCrotti#387).

snippets/prog-mode/commentblock

@@ -15,15 +15,20 @@ ${1:$(let* ((col (current-column))
 ${1:$(let* ((col (current-column))
            (str "")
            (start (yas-trimmed-comment-start))
+           (start (if (string= start ";") ";;" start))
            (end (yas-trimmed-comment-end)))
          (while (< (length str) (ffloor (/ (- 78.0 (+ col (length start) (string-width yas-text) (length end))) 2.0)))
                 (setq str (concat str " ")))
-        (concat start str))} ${1:comment} ${1:$(let* ((col (current-column))
-                                                     (str "")
-                                                     (start (yas-trimmed-comment-start))
-                                                     (end (yas-trimmed-comment-end)))
-                                                   (while (< (length str) (- 79.0 (if (eq (mod (string-width yas-text) 2) 1) (- col 1) col) (length end)))
-                                                          (setq str (concat str " ")))
+        (concat start str))} ${1:comment} ${1:$(
+        let* ((col (current-column))
+              (str "")
+              (start (yas-trimmed-comment-start))
+              (start (if (string= start ";") ";;" start))
+              (end (yas-trimmed-comment-end)))
+              (while (< (length str)
+                        (- 79 (length end)
+                           (if (eq (mod (+ (string-width yas-text) (length start) (length end)) 2) 1) (- col 1) col)))
+              (setq str (concat str " ")))
                                                  (concat str end))}
 ${1:$(let* ((col (current-column))
            (str "")