Diff includes don't always work reliably

[NathanLovato]

We use a leading + or - sign on an include line to indicate that an included code block should have + or - signs prefixed to each line, for diff display.

In this example the fourth include fails and includes the entire file, ignoring the anchor attribute:

<Include file="bouncer.gd" anchor="physics_process_signature"/>
-<Include file="07_runner.gd" anchor="direction_calculation"/>
+<Include file="bouncer.gd" anchor="direction"/>
<Include file="bouncer.gd" anchor="desired_velocity"/>

On top of fixing this, we may consider making the diff aspect an attribute, like diff="+", as suggested by Jad, instead of prepending the prefix in the line, for parsing and writing consistency. This would be part of #65

[NathanLovato]

The bug was something else; if an anchor is missing an END marker, the parser includes the entire file's content. Opening a separate ticket.