avoid looking at errno for short read_in_full() returns

avoid looking at errno for short read_in_full() returns

When a caller tries to read a particular set of bytes via
read_in_full(), there are three possible outcomes:

  1. An error, in which case -1 is returned and errno is
     set.

  2. A short read, in which fewer bytes are returned and
     errno is unspecified (we never saw a read error, so we
     may have some random value from whatever syscall failed
     last).

  3. The full read completed successfully.

Many callers handle cases 1 and 2 together by just checking
the result against the requested size. If their combined
error path looks at errno (e.g., by calling die_errno), they
may report a nonsense value.

Let's fix these sites by having them distinguish between the
two error cases. That avoids the random errno confusion, and
lets us give more detailed error messages.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>