progress: use xmalloc/xcalloc

progress: use xmalloc/xcalloc

Since the early days of Git, the progress code allocates its struct with
a bare malloc(), not xmalloc(). If the allocation fails, we just avoid
showing progress at all.

While perhaps a noble goal not to fail the whole operation because of
optional progress, in practice:

  1. Any failure to allocate a few dozen bytes here means critical path
     allocations are likely to fail, too.

  2. These days we use a strbuf for throughput progress (and there's a
     patch under discussion to do the same for non-throughput cases,
     too). And that uses xmalloc() under the hood, which means we'd
     still die on some allocation failures.

Let's switch to xmalloc(). That makes us consistent with the rest of Git
and makes it easier to audit for other (less careful) bare mallocs.

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