add a function to get file content from db segment
[BUG_FIX] a bug in code fragmnet view for processing fragment change was fixed
[BUG_FIX] fix wrong bug fix
[REFACTORING] collect a mis-leading variable name
[BUG_FIX] fix a bug related to identify ghost fragments
add a field to represent whether code fragments are ghost or not
remove the field of ghost fragments
add a method in CodeFragment to get whether the fragment is ghost