largefiles: reuse hexsha1() to centralize hash calculation logic into it This patch also renames argument of hexsha1(), not only for readability ("data" isn't good name for file-like object), but also for reviewability (including hexsha1() code helps reviewers to confirm how these functions are similar). BTW, copyandhash() has also similar logic, but it can't reuse hexsha1(), because it writes read-in data into specified fileobj simultaneously.