grep: remove the kwset optimization

grep: remove the kwset optimization

A later change will replace this optimization with optimistic use of
PCRE v2. I'm completely removing it as an intermediate step, as
opposed to replacing it with PCRE v2, to demonstrate that no grep
semantics depend on this (or any other) optimization for the fixed
backend anymore.

For now this is mostly (but not entirely) a performance regression, as
shown by this hacky one-liner:

    for opt in '' ' -i'
        do
        GIT_PERF_7821_GREP_OPTS=$opt GIT_PERF_REPEAT_COUNT=10 GIT_PERF_LARGE_REPO=~/g/linux GIT_PERF_MAKE_OPTS='-j8 CFLAGS=-O3 USE_LIBPCRE=YesPlease' ./run origin/master HEAD -- p7821-grep-engines-fixed.sh
    done &&
    for opt in '' ' -i'
        do GIT_PERF_4221_LOG_OPTS=$opt GIT_PERF_REPEAT_COUNT=10 GIT_PERF_LARGE_REPO=~/g/linux GIT_PERF_MAKE_OPTS='-j8 CFLAGS=-O3 USE_LIBPCRE=YesPlease' ./run origin/master HEAD -- p4221-log-grep-engines-fixed.sh
    done

Which produces:

plain grep:

    Test                             origin/master     HEAD
    -------------------------------------------------------------------------
    7821.1: fixed grep int           0.55(1.60+0.63)   0.82(3.11+0.51) +49.1%
    7821.2: basic grep int           0.62(1.68+0.49)   0.85(3.02+0.52) +37.1%
    7821.3: extended grep int        0.61(1.63+0.53)   0.91(3.09+0.44) +49.2%
    7821.4: perl grep int            0.55(1.60+0.57)   0.41(0.93+0.57) -25.5%
    7821.6: fixed grep uncommon      0.20(0.50+0.44)   0.35(1.27+0.42) +75.0%
    7821.7: basic grep uncommon      0.20(0.49+0.45)   0.35(1.29+0.41) +75.0%
    7821.8: extended grep uncommon   0.20(0.45+0.48)   0.35(1.25+0.44) +75.0%
    7821.9: perl grep uncommon       0.20(0.53+0.41)   0.16(0.24+0.49) -20.0%
    7821.11: fixed grep æ            0.35(1.27+0.40)   0.25(0.82+0.39) -28.6%
    7821.12: basic grep æ            0.35(1.28+0.38)   0.25(0.75+0.44) -28.6%
    7821.13: extended grep æ         0.36(1.21+0.46)   0.25(0.86+0.35) -30.6%
    7821.14: perl grep æ             0.35(1.33+0.34)   0.16(0.26+0.47) -54.3%

grep with -i:

    Test                                origin/master     HEAD
    -----------------------------------------------------------------------------
    7821.1: fixed grep -i int           0.61(1.84+0.64)   1.11(4.12+0.64) +82.0%
    7821.2: basic grep -i int           0.72(1.86+0.57)   1.15(4.48+0.49) +59.7%
    7821.3: extended grep -i int        0.94(1.83+0.60)   1.53(4.12+0.58) +62.8%
    7821.4: perl grep -i int            0.66(1.82+0.59)   0.55(1.08+0.58) -16.7%
    7821.6: fixed grep -i uncommon      0.21(0.51+0.44)   0.44(1.74+0.34) +109.5%
    7821.7: basic grep -i uncommon      0.21(0.55+0.41)   0.44(1.72+0.40) +109.5%
    7821.8: extended grep -i uncommon   0.21(0.57+0.39)   0.42(1.64+0.45) +100.0%
    7821.9: perl grep -i uncommon       0.21(0.48+0.48)   0.17(0.30+0.45) -19.0%
    7821.11: fixed grep -i æ            0.25(0.73+0.45)   0.25(0.75+0.45) +0.0%
    7821.12: basic grep -i æ            0.25(0.71+0.49)   0.26(0.77+0.44) +4.0%
    7821.13: extended grep -i æ         0.25(0.75+0.44)   0.25(0.74+0.46) +0.0%
    7821.14: perl grep -i æ             0.17(0.26+0.48)   0.16(0.20+0.52) -5.9%

plain log:

    Test                                     origin/master     HEAD
    ---------------------------------------------------------------------------------
    4221.1: fixed log --grep='int'           7.31(7.06+0.21)   8.11(7.85+0.20) +10.9%
    4221.2: basic log --grep='int'           7.30(6.94+0.27)   8.16(7.89+0.19) +11.8%
    4221.3: extended log --grep='int'        7.34(7.05+0.21)   8.08(7.76+0.25) +10.1%
    4221.4: perl log --grep='int'            7.27(6.94+0.24)   7.05(6.76+0.25) -3.0%
    4221.6: fixed log --grep='uncommon'      6.97(6.62+0.32)   7.86(7.51+0.30) +12.8%
    4221.7: basic log --grep='uncommon'      7.05(6.69+0.29)   7.89(7.60+0.28) +11.9%
    4221.8: extended log --grep='uncommon'   6.89(6.56+0.32)   7.99(7.66+0.24) +16.0%
    4221.9: perl log --grep='uncommon'       7.02(6.66+0.33)   6.97(6.54+0.36) -0.7%
    4221.11: fixed log --grep='æ'            7.37(7.03+0.33)   7.67(7.30+0.31) +4.1%
    4221.12: basic log --grep='æ'            7.41(7.00+0.31)   7.60(7.28+0.26) +2.6%
    4221.13: extended log --grep='æ'         7.35(6.96+0.38)   7.73(7.31+0.34) +5.2%
    4221.14: perl log --grep='æ'             7.43(7.10+0.32)   6.95(6.61+0.27) -6.5%

log with -i:

    Test                                        origin/master     HEAD
    ------------------------------------------------------------------------------------
    4221.1: fixed log -i --grep='int'           7.40(7.05+0.23)   8.66(8.38+0.20) +17.0%
    4221.2: basic log -i --grep='int'           7.39(7.09+0.23)   8.67(8.39+0.20) +17.3%
    4221.3: extended log -i --grep='int'        7.29(6.99+0.26)   8.69(8.31+0.26) +19.2%
    4221.4: perl log -i --grep='int'            7.42(7.16+0.21)   7.14(6.80+0.24) -3.8%
    4221.6: fixed log -i --grep='uncommon'      6.94(6.58+0.35)   8.43(8.04+0.30) +21.5%
    4221.7: basic log -i --grep='uncommon'      6.95(6.62+0.31)   8.34(7.93+0.32) +20.0%
    4221.8: extended log -i --grep='uncommon'   7.06(6.75+0.25)   8.32(7.98+0.31) +17.8%
    4221.9: perl log -i --grep='uncommon'       6.96(6.69+0.26)   7.04(6.64+0.32) +1.1%
    4221.11: fixed log -i --grep='æ'            7.92(7.55+0.33)   7.86(7.44+0.34) -0.8%
    4221.12: basic log -i --grep='æ'            7.88(7.49+0.32)   7.84(7.46+0.34) -0.5%
    4221.13: extended log -i --grep='æ'         7.91(7.51+0.32)   7.87(7.48+0.32) -0.5%
    4221.14: perl log -i --grep='æ'             7.01(6.59+0.35)   6.99(6.64+0.28) -0.3%

Some of those, as noted in [1] are because PCRE is faster at finding
fixed strings. This looks bad for some engines, but in the next change
we'll optimistically use PCRE v2 for all of these, so it'll look
better.

1. https://public-inbox.org/git/87v9x793qi.fsf@evledraar.gmail.com/

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