Only write "diff" instead of full ClassPath to class cache file

[ZZZank]

This can greatly reduce the size of .probe/classes.txt. Example:

com.electronwill.nightconfig.core.CommentedConfig	3
....CommentedConfig$Entry	4
....Config	4
....Config$Entry	5
....ConfigFormat	4
....EnumGetMethod	4
....UnmodifiableCommentedConfig	3

Dots at the starting of ClassPath means the number of common package parts this ClassPath shares with the previous ClassPath:

com.electronwill.nightconfig.core.CommentedConfig
   .            .           .    .CommentedConfig$Entry
   .            .           .    .Config