changes_comments should have compound foreign key (file_id, hash)

[nanokatze]

There can exist same change hash for two or more different files. Example:

0. you open two files with exactly same content
1. their base changes are both 0
2. erase first 4 bytes in both
3. change hash is sha256(base + file_pos + change_type + data)
4. get collision

[nyxxxie]

@fst3a is this fixed with sqlalchemy changes in feature/implement_project?

[nanokatze]

@nyxxxie no.