grep threading cleanup & tests

This is the spiritual successor to the "grep: add ability to disable threading with --threads=0 or grep.threads=0" patch I submitted as part of my PCRE series (<[email protected]>).

There's a long back & forth thread between me and Jeff King as a follow-up to that which I'll summarize: I'd misunderstood what --threads=1 does, because I was tracing how often a pattern was compiled, and observing that even with --threads=1 we'd compile the pattern twice.

This series fixes that redundant needless compilation in 3/8, skips pthread overhead as Jeff suggested kin 3/8. The rest of the patches are a bunch of small related fixes I noticed along the way.

Ævar Arnfjörð Bjarmason (8):
grep: assert that threading is enabled when calling grep_{lock,unlock}
grep: add tests for --threads=N and grep.threads
grep: don't redundantly compile throwaway patterns under threading
grep: skip pthreads overhead when using one thread
tests: add a PTHREADS prerequisite
pack-object & index-pack: add test for --threads warning under
NO_PTHREADS
pack-objects: fix buggy warning about threads under
NO_PTHREADS=YesPlease
grep: given --threads with NO_PTHREADS=YesPlease, warn

 Makefile               |  1 +
 builtin/grep.c         | 32 +++++++++++++++++++++++++-------
 builtin/pack-objects.c |  4 +++-
 t/README               |  4 ++++
 t/t5300-pack-object.sh | 33 +++++++++++++++++++++++++++++++++
 t/t7810-grep.sh        | 34 ++++++++++++++++++++++++++++++++++
 t/test-lib.sh          |  1 +
 7 files changed, 101 insertions(+), 8 deletions(-)

2.11.0