Make git --graph option optional again in log washing.

The (newer) "magit-log-oneline-re" regular expression was requiring
--graph, making the (older) magit-configure-have-graph logic
irrelevant: it had to return "true". Making --graph optional again is
not just fixing support for old git clients but also opening some log
possibilities, like removing the spurious star or showing tags in the
status view.

magit.el

@@ -2422,7 +2422,7 @@ in the corresponding directories."
 
 (defvar magit-log-oneline-re
   (concat
-   "^\\([_\\*|/ -.]+\\)"         ; graph   (1)
+   "^\\([_\\*|/ -.]+\\)?"        ; graph   (1)
    "\\(?:"
    "\\([0-9a-fA-F]\\{40\\}\\) "  ; sha1    (2)
    "\\(?:\\((.+?)\\) \\)?"       ; refs    (3)
@@ -2468,7 +2468,8 @@ must return a string which will represent the log line.")
 	 (propertize (substring sha1 0 8) 'face 'magit-log-sha1)
        (insert-char ? 8))
      " "
-     (propertize graph 'face 'magit-log-graph)
+     (when graph
+       (propertize graph 'face 'magit-log-graph))
      string-refs
      (when message
        (propertize message 'face 'magit-log-message)))))