Fix fetch-clone in the presense of signals

Fix fetch-clone in the presense of signals

We shouldn't fail a fetch just because a signal might have interrupted
the read.

Normally, we don't install any signal handlers, so EINTR really shouldn't
happen. That said, really old versions of Linux will interrupt an
interruptible system call even for signals that turn out to be ignored
(SIGWINCH is the classic example - resizing your xterm would cause it).
The same might well be true elsewhere too.

Also, since receive_keep_pack() doesn't control the caller, it can't know
that no signal handlers exist.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>