Allow multiple calls to VundleLog.

If it was loaded, unload the log buffer before editing it.  Otherwise the
editing command can "hang" if the user has `set hidden`.

This problem was originally discovered with the VundleChangelog command and
the analogous fix was applied in 7d9b10.  See github issue #468 for more.

autoload/vundle/scripts.vim

@@ -58,6 +58,9 @@ func! s:view_log()
     let s:log_file = tempname()
   endif
 
+  if bufloaded(s:log_file)
+    execute 'silent bdelete' s:log_file
+  endif
   call writefile(g:vundle_log, s:log_file)
   execute 'silent pedit ' . s:log_file