pickaxe -G: don't special-case create/delete

Instead of special-casing creations and deletions let's just generate
a diff for them.

This logic of not running a diff under -G if we don't have both sides
dates back to the original implementation of -S in
52e9578985f ([PATCH] Introducing software archaeologist's tool
"pickaxe"., 2005-05-21).

In the case of -S we were not working with the xdiff interface and
needed to do this, but when -G was implemented in f506b8e8b5f (git
log/diff: add -G<regexp> that greps in the patch text, 2010-08-23)
this logic was diligently copied over.

But as the performance test added earlier in this series shows, this
does not make much of a difference. With:

    time GIT_TEST_LONG= GIT_PERF_REPEAT_COUNT=10 GIT_PERF_MAKE_OPTS='-j8 CFLAGS=-O3' ./run origin/next HEAD~ HEAD -- p4209-pickaxe.sh

With the HEAD~ commit being the preceding "pickaxe -G: terminate early
on matching lines" we get these results. Note that it's only the -G
codepaths that are relevant to this change:

    Test                                                                      origin/next       HEAD~                   HEAD
    -----------------------------------------------------------------------------------------------------------------------------------------
    4209.1: git log -S'int main' <limit-rev>..                                0.35(0.32+0.03)   0.35(0.33+0.02) +0.0%   0.35(0.30+0.05) +0.0%
    4209.2: git log -S'æ' <limit-rev>..                                       0.46(0.42+0.04)   0.46(0.41+0.05) +0.0%   0.46(0.42+0.04) +0.0%
    4209.3: git log --pickaxe-regex -S'(int|void|null)' <limit-rev>..         0.65(0.62+0.02)   0.64(0.61+0.02) -1.5%   0.64(0.60+0.04) -1.5%
    4209.4: git log --pickaxe-regex -S'if *\([^ ]+ & ' <limit-rev>..          0.52(0.45+0.06)   0.52(0.50+0.01) +0.0%   0.54(0.47+0.04) +3.8%
    4209.5: git log --pickaxe-regex -S'[àáâãäåæñøùúûüýþ]' <limit-rev>..       0.39(0.34+0.05)   0.39(0.34+0.04) +0.0%   0.39(0.36+0.03) +0.0%
    4209.6: git log -G'(int|void|null)' <limit-rev>..                         0.60(0.55+0.04)   0.58(0.54+0.03) -3.3%   0.58(0.49+0.08) -3.3%
    4209.7: git log -G'if *\([^ ]+ & ' <limit-rev>..                          0.61(0.52+0.06)   0.59(0.53+0.05) -3.3%   0.59(0.54+0.05) -3.3%
    4209.8: git log -G'[àáâãäåæñøùúûüýþ]' <limit-rev>..                       0.61(0.51+0.07)   0.58(0.54+0.04) -4.9%   0.57(0.51+0.06) -6.6%
    4209.9: git log -i -S'int main' <limit-rev>..                             0.36(0.31+0.04)   0.36(0.34+0.02) +0.0%   0.35(0.32+0.03) -2.8%
    4209.10: git log -i -S'æ' <limit-rev>..                                   0.36(0.33+0.03)   0.39(0.34+0.01) +8.3%   0.36(0.32+0.03) +0.0%
    4209.11: git log -i --pickaxe-regex -S'(int|void|null)' <limit-rev>..     0.83(0.77+0.05)   0.82(0.77+0.05) -1.2%   0.80(0.75+0.04) -3.6%
    4209.12: git log -i --pickaxe-regex -S'if *\([^ ]+ & ' <limit-rev>..      0.67(0.61+0.03)   0.64(0.61+0.03) -4.5%   0.63(0.61+0.02) -6.0%
    4209.13: git log -i --pickaxe-regex -S'[àáâãäåæñøùúûüýþ]' <limit-rev>..   0.40(0.37+0.02)   0.40(0.37+0.03) +0.0%   0.40(0.36+0.04) +0.0%
    4209.14: git log -i -G'(int|void|null)' <limit-rev>..                     0.58(0.51+0.07)   0.59(0.52+0.06) +1.7%   0.58(0.52+0.05) +0.0%
    4209.15: git log -i -G'if *\([^ ]+ & ' <limit-rev>..                      0.60(0.54+0.05)   0.60(0.54+0.06) +0.0%   0.60(0.56+0.03) +0.0%
    4209.16: git log -i -G'[àáâãäåæñøùúûüýþ]' <limit-rev>..                   0.58(0.51+0.06)   0.57(0.52+0.05) -1.7%   0.60(0.48+0.09) +3.4%

This small simplification really doesn't buy us much now, but I've got
plans to both convert the pickaxe code to using a PCREv2 backend[1]
and to implement additional pickaxe modes to do custom searches
through the diff[2]. Always having the diff available under -G is
going to help to simplify both of those changes.

1. https://lore.kernel.org/git/20210203032811.14979-22-avarab@gmail.com/
2. https://lore.kernel.org/git/20190424152215.16251-3-avarab@gmail.com/

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

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