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.