coccicheck: optionally batch spatch invocations

In our "make coccicheck" rule, we currently feed each source file to its
own individual invocation of spatch. This has a few downsides:

  - it repeats any overhead spatch has for starting up and reading the
    patch file

  - any included header files may get processed from multiple
    invocations. This is slow (we see the same header files multiple
    times) and may produce a resulting patch with repeated hunks (which
    cannot be applied without further cleanup)

Ideally we'd just invoke a single instance of spatch per rule-file and
feed it all source files. But spatch can be rather memory hungry when
run in this way. I measured the peak RSS going from ~90MB for a single
file to ~1900MB for all files. Multiplied by multiple rule files being
processed at the same time (for "make -j"), this can make things slower
or even cause them to fail (e.g., this is reported to happen on our
Travis builds).

Instead, let's provide a tunable knob. We'll leave the default at "1",
but it can be cranked up to "999" for maximum CPU/memory tradeoff, or
people can find points in between that serve their particular machines.

Here are a few numbers running a single rule via:

  SIZES='1 4 16 999'
  RULE=contrib/coccinelle/object_id.cocci
  for i in $SIZES; do
    make clean
    /usr/bin/time -o $i.out --format='%e | %U | %S | %M' \
      make $RULE.patch SPATCH_BATCH_SIZE=$i
  done
  for i in $SIZES; do
    printf '%4d | %s\n' $i "$(cat $i.out)"
  done

which yields:

     1 | 97.73 | 93.38 | 4.33 | 100128
     4 | 52.80 | 51.14 | 1.69 | 135204
    16 | 35.82 | 35.09 | 0.76 | 284124
   999 | 23.30 | 23.13 | 0.20 | 1903852

The implementation is done with xargs, which should be widely available;
it's in POSIX, we rely on it already in the test suite. And "coccicheck"
is really a developer-only tool anyway, so it's not a big deal if
obscure systems can't run it.

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

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