perf: add a comparison test of log --grep regex engines with -F

perf: add a comparison test of log --grep regex engines with -F

Add a performance comparison test of log --grepgrep regex engines
given fixed strings.

See the preceding fixed-string t/perf change ("perf: add a comparison
test of grep regex engines with -F", 2017-04-21) for notes about this,
in particular this mostly tests exactly the same codepath now, but
might not in the future:

    $ GIT_PERF_REPEAT_COUNT=10 GIT_PERF_LARGE_REPO=~/g/linux ./run p4221-log-grep-engines-fixed.sh
    [...]
    Test                                     this tree
    --------------------------------------------------------
    4221.1: fixed log --grep='int'           5.99(5.55+0.40)
    4221.2: basic log --grep='int'           5.92(5.56+0.31)
    4221.3: extended log --grep='int'        6.01(5.51+0.45)
    4221.4: perl log --grep='int'            5.99(5.56+0.38)
    4221.6: fixed log --grep='uncommon'      5.06(4.76+0.27)
    4221.7: basic log --grep='uncommon'      5.02(4.78+0.21)
    4221.8: extended log --grep='uncommon'   4.99(4.78+0.20)
    4221.9: perl log --grep='uncommon'       5.00(4.72+0.26)
    4221.11: fixed log --grep='æ'            5.35(5.12+0.20)
    4221.12: basic log --grep='æ'            5.34(5.11+0.20)
    4221.13: extended log --grep='æ'         5.39(5.10+0.22)
    4221.14: perl log --grep='æ'             5.44(5.16+0.23)

Only the non-ASCII -i case is different:

    $ GIT_PERF_REPEAT_COUNT=10 GIT_PERF_LARGE_REPO=~/g/linux GIT_PERF_4221_LOG_OPTS=' -i' ./run p4221-log-grep-engines-fixed.sh
    [...]
    Test                                        this tree
    -----------------------------------------------------------
    4221.1: fixed log -i --grep='int'           6.17(5.77+0.35)
    4221.2: basic log -i --grep='int'           6.16(5.59+0.39)
    4221.3: extended log -i --grep='int'        6.15(5.70+0.39)
    4221.4: perl log -i --grep='int'            6.15(5.69+0.38)
    4221.6: fixed log -i --grep='uncommon'      5.10(4.88+0.21)
    4221.7: basic log -i --grep='uncommon'      5.04(4.76+0.25)
    4221.8: extended log -i --grep='uncommon'   5.07(4.82+0.23)
    4221.9: perl log -i --grep='uncommon'       5.03(4.78+0.22)
    4221.11: fixed log -i --grep='æ'            5.93(5.65+0.25)
    4221.12: basic log -i --grep='æ'            5.88(5.62+0.25)
    4221.13: extended log -i --grep='æ'         6.02(5.69+0.29)
    4221.14: perl log -i --grep='æ'             5.36(5.06+0.29)

See commit ("perf: add a comparison test of grep regex engines",
2017-04-19) for details on the machine the above test run was executed
on.

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