Fixed issue #2326: Set "follow renames" as default for logs of single files

Open username-removed-217046 requested to merge hpavlov/tortoisegit:issue_2326 into master

Other work done in this commit:

  • Added a new helper method TGitPath::IsFile()
  • Fixed the "Walk Behaviour" menu items to be greyed our when disabled so it is clear you can't click then
  • Small refactor around the hiding of the graph column