Fixed bug #50: https://github.com/leo-editor/leo-editor/issues/50 Important: body text lost switching @file to @auto-rst
This case is so dangerous that Leo refuses to create the file and instead reverts to the previous @file node.
- v.at_read is now a dict: keys are full file names; values are lists of headlines.
- at.shouldPromptForDangerousWrite returns True if p.h is not in v.at_read.
- at.promptForDangerousWrite issues a warning and reverts the headline

Leo build: 20141018080627