PCRE v1 improvements & PCRE v2 support

Trivial changes since last time. Just sending this because I'd like the copy in pu updated. Changes noted below:

Ævar Arnfjörð Bjarmason (19):
grep: amend submodule recursion test in preparation for rx engine
testing
grep: add tests for grep pattern types being passed to submodules

A s/PCRE/LIBPCRE/ on the test_have_prereq, now makes sense with the series in sequence (error added during rebasing).

grep: submodule-related case statements should die if new fields are
added
grep: remove redundant regflags assignment under PCRE
grep: remove redundant regflags &= ~REG_EXTENDED assignments

NEW: Similarly to how we didn't need to set regflags under PCRE, we were negating REG_EXTENDED under POSIX basic, without ever setting it in the first place.

This was just as confusing as the PCRE oddity, so remove it.

Makefile & configure: reword outdated comment about PCRE
grep: add a test for backreferences in PCRE patterns
log: add exhaustive tests for pattern style options & config
log: add -P as a synonym for --perl-regexp
grep & rev-list doc: stop promising libpcre for --perl-regexp
grep: make grep.patternType=[pcre|pcre1] a synonym for "perl"
test-lib: rename the LIBPCRE prerequisite to PCRE

One corresponding s/LIBPCRE/PCRE/ for the earlier change, see above.

grep: change the internal PCRE macro names to be PCRE1
grep: change the internal PCRE code & header names to be PCRE1
perf: add a performance comparison test of grep -E and -P
grep: add support for the PCRE v1 JIT API
grep: add support for PCRE v2

We now give proper error messages via pcre2_get_error_message() when pcre2_match() fails with errors other than "didn't match", the common case for this is that the engine gave up on a pathological pattern / input combination.

grep: remove support for concurrent use of both PCRE v1 & v2 Makefile & configure: make PCRE v2 the default PCRE implementation

Added more details to the commit message about why switching to PCRE v2 by default is a good idea. I hadn't noticed before that deep bugs in PCRE v1 are being WONTFIX'd on the bugtracker saying "nope, never fixing thath in v1, switch to v2".

 Documentation/git-grep.txt         |   7 +-
 Documentation/rev-list-options.txt |   9 +-
 Makefile                           |  39 +++++--
 builtin/grep.c                     |   4 +
 configure.ac                       |  81 ++++++++++++--
 grep.c                             | 222 +++++++++++++++++++++++++++++++------
 grep.h                             |  32 +++++-
 revision.c                         |   2 +-
 t/README                           |   4 +-
 t/perf/p7820-grep-engines.sh       |  25 +++++
 t/t4202-log.sh                     |  86 +++++++++++++-
 t/t7810-grep.sh                    |  41 ++++---
 t/t7812-grep-icase-non-ascii.sh    |   4 +-
 t/t7813-grep-icase-iso.sh          |  11 +-
 t/t7814-grep-recurse-submodules.sh | 215 +++++++++++++++++++++--------------
 t/test-lib.sh                      |   3 +-
 16 files changed, 613 insertions(+), 172 deletions(-)
 create mode 100755 t/perf/p7820-grep-engines.sh

2.11.0