Blame file at specific commit

Add an option to Blame Selected at a specific commit, in order to show the file at that specific commit rather than the current state on disk.

Useful if later commits have overwritten text which you wanted to trace the source of, or if local changes were overriding them (as in https://answers.atlassian.com/questions/158417/blame-in-sourcetree-for-windows).

Feature should be available from the context menu on files in commit details in the log and in the File Log window.