Add History option for folders

Similar to SRCTREEWIN-5502, but it's to add a context menu option to show the history of a folder. Currently you can do a Git Log on an individual file, but you can't on a folder (including it's subfolders/files).

SourceTree does come with gitk, which can view the histories of folders though. A custom action could be implemented that passes in a new $PATH token which would open gitk – $PATH ($PATH being the currently selected folder or file path, similar to $FILE). It would be better though if this didn't have to be a custom action and it could just be implemented as a new context menu option when right-clicking a file/folder (such as "View history in Gitk").