clone: clean up directory after transport_fetch_refs() failure

git-clone started respecting errors from the transport subsystem in
aab179d937 (builtin/clone.c: don't ignore transport_fetch_refs() errors,
2020-12-03). However, that commit didn't handle the cleanup of the
filesystem quite right.

The cleanup of the directory that cmd_clone() creates is done by an
atexit() handler, which we control with a flag. It starts as
JUNK_LEAVE_NONE ("clean up everything"), then progresses to
JUNK_LEAVE_REPO when we know we have a valid repo but not working tree,
and then finally JUNK_LEAVE_ALL when we have a successful checkout.

Most errors cause us to die(), which then triggers the handler to do the
right thing based on how far into cmd_clone() we got. But the checks
added by aab179d937 instead set the "err" variable and then jump to a
new "cleanup" label, which then returns our non-zero status. However,
the code after the cleanup label includes setting the flag to
JUNK_LEAVE_ALL, and so we accidentally leave the repository and working
tree in place.

One obvious option to fix this is to reorder the end of the function to
set the flag first, before cleanup code, and put the label between them.

But we can observe another small bug: the error return from
transport_fetch_refs() is generally "-1", and we propagate that to the
return value of cmd_clone(), which ultimately becomes the exit code of
the process. And we try to avoid transmitting negative values via exit
codes (only the low 8 bits are passed along as an unsigned value, though
in practice for "-1" this at least retains the property that it's
non-zero).

Instead, let's just die(). That makes us consistent with rest of the
code in the function. It does add a new "fatal:" line to the output, but
I'd argue that's a good thing:

  - in the rare case that the transport code didn't say anything, now
    the user gets _some_ error message

  - even if the transport code said something like "error: ssh died of
    signal 9", it's nice to also say "fatal" to indicate that we
    considered that to be a show-stopper.

Triggering this in the test suite turns out to be surprisingly
difficult. Almost every error we'd encounter, including ones deep inside
the transport code, cause us to just die() right there! However, one way
is to put a fake wrapper around git-upload-pack that sends the complete
packfile but exits with a failure code.

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

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