grep/pcre2: actually make pcre2 use custom allocator

Continue work started in 513f2b0bbd4 (grep: make PCRE2 aware of custom
allocator, 2019-10-16) and make PCREv2 use our pcre2_{malloc,free}().
functions for allocation. We'll now use it for all PCREv2 allocations.

The reason 513f2b0bbd4 worked as a bugfix for the USE_NED_ALLOCATOR
issue is because it targeted the allocation freed via free(), as
opposed to by a pcre2_*free() function. I.e. the pcre2_maketables()
and pcre2_maketables_free() pair.

For most of the rest we continued allocating with stock malloc()
inside PCREv2 itself, but didn't segfault because we'd use its
corresponding free().

In a preceding commit of mine I changed the free() to
pcre2_maketables_free() on versions of PCREv2 10.34 and newer. So as
far as fixing the segfault goes we could revert 513f2b0bbd4. But then
we wouldn't use the desired allocator, let's just use it instead.

Before this patch we'd on e.g.:

    grep --threads=1 -iP æ.*var.*xyz

Only use pcre2_{malloc,free}() for 2 malloc() calls and 2
corresponding free() calls. Now it's 12 calls to each. This can be
observed with the GREP_PCRE2_DEBUG_MALLOC debug mode.

Reading the history of how this bug got introduced it wasn't present
in Johannes's original patch[1] to fix the issue.

My reading of that thread is that the approach the follow-up patches
to Johannes's original pursued were based on misunderstanding of how
the PCREv2 API works. In particular this part of [2]:

    "most of the time (like when using UTF-8) the chartable (and
    therefore the global context) is not needed (even when using
    alternate allocators)"

That's simply not how PCREv2 memory allocation works. It's easy to see
how the misunderstanding came about. It's because (as noted above) the
issue was noticed because of our use of free() in our own grep.c for
freeing the memory allocated by pcre2_maketables().

Thus the misunderstanding that PCREv2's compile context is something
only needed for pcre2_maketables(), and e.g. an aborted earlier
attempt[3] to only set it up when we ourselves called
pcre2_maketables().

That's not what PCREv2's compile context is. To quote PCREv2's
documentation:

    "This context just contains pointers to (and data for) external
    memory management functions that are called from several places in
    the PCRE2 library."

Thus the failed attempts to go down the route of only creating the
general context in cases where we ourselves call pcre2_maketables(),
before finally settling on the approach 513f2b0bbd4 took of always
creating it, but then mostly not using it.

Instead we should always create it, and then pass the general context
to those functions that accept it, so that they'll consistently use
our preferred memory allocation functions.

1. https://lore.kernel.org/git/3397e6797f872aedd18c6d795f4976e1c579514b.1565005867.git.gitgitgadget@gmail.com/
2. https://lore.kernel.org/git/CAPUEsphMh_ZqcH3M7PXC9jHTfEdQN3mhTAK2JDkdvKBp53YBoA@mail.gmail.com/
3. https://lore.kernel.org/git/20190806085014.47776-3-carenas@gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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