sha1_file.c: cleanup "offset" usage

sha1_file.c: cleanup "offset" usage

First there are too many offsets there and it is getting confusing.
So 'offset' is now 'curpos' to distinguish from other offsets like
'obj_offset'.

Then structures like x = foo(x, &y) are now done as y = foo(&x).
It looks more natural that the result y be returned directly and
x be passed as reference to be updated in place.  This has the effect
of reducing some line length and removing a few, needing a bit less
stack space, and it even reduces the compiled code size.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>