Get the relative path of fileName and folderName
fileName = c:\abc\def\foo.java
folderName = c:\abc
relativeName = def\foo.java
This function will not handle Unix/Windows path separator conversation, but will append a java.io.File.separator if folderName does not end with one
There are two things we need to check
files created or modified since last build time, we only need to check the source folder
file deleted since last build time, we have to compare source and destination folder