magit-emacsclient-executable: try more version suffixed names
add support for using unicode characters when drawing log graphs

Add new function `magit-log-format-unicode-graph' and add it as a
choice in option `magit-log-format-graph-function'.  Also add new
option `magit-log-format-unicode-graph-alist' allowing users to
choose the characters (unicode or not, actually) to be used.