title: Is there a method to find a file by hash in JGit
body: <p>I am coding a Java program with JGit to handle the local Git repository. I am wondering if there is a way to find a committed file via hash in JGit like what the command git file-cat -p  does in the terminal? To be specific, given the hash of a file, the program should return the file content if it has been committed before, otherwise, return null. the file to be retrieved can be from any previous commit.</p>