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

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

Add a performance comparison test of grep regex engines given fixed
strings.

The current logic in compile_regexp() ignores the engine parameter and
uses kwset() to search for these, so this test shows no difference
between engines right now:

    $ GIT_PERF_REPEAT_COUNT=10 GIT_PERF_LARGE_REPO=~/g/linux ./run p7821-grep-engines-fixed.sh
    [...]
    Test                             this tree
    ------------------------------------------------
    7821.1: fixed grep int           0.56(1.67+0.68)
    7821.2: basic grep int           0.57(1.70+0.57)
    7821.3: extended grep int        0.59(1.76+0.51)
    7821.4: perl grep int            1.08(1.71+0.55)
    7821.6: fixed grep uncommon      0.23(0.55+0.50)
    7821.7: basic grep uncommon      0.24(0.55+0.50)
    7821.8: extended grep uncommon   0.26(0.55+0.52)
    7821.9: perl grep uncommon       0.24(0.58+0.47)
    7821.11: fixed grep æ            0.36(1.30+0.42)
    7821.12: basic grep æ            0.36(1.32+0.40)
    7821.13: extended grep æ         0.38(1.30+0.42)
    7821.14: perl grep æ             0.35(1.24+0.48)

Only when run with -i via GIT_PERF_7821_GREP_OPTS=' -i' do we avoid
avoid going through the same kwset.[ch] codepath, see the "Even when
-F..."  comment in grep.c. This only kicks for the non-ASCII case:

    $ GIT_PERF_REPEAT_COUNT=10 GIT_PERF_LARGE_REPO=~/g/linux GIT_PERF_7821_GREP_OPTS=' -i' ./run p7821-grep-engines-fixed.sh
    [...]
    Test                                this tree
    ---------------------------------------------------
    7821.1: fixed grep -i int           0.62(2.10+0.57)
    7821.2: basic grep -i int           0.68(1.90+0.61)
    7821.3: extended grep -i int        0.78(1.94+0.57)
    7821.4: perl grep -i int            0.98(1.78+0.74)
    7821.6: fixed grep -i uncommon      0.24(0.44+0.64)
    7821.7: basic grep -i uncommon      0.25(0.56+0.54)
    7821.8: extended grep -i uncommon   0.27(0.62+0.45)
    7821.9: perl grep -i uncommon       0.24(0.59+0.49)
    7821.11: fixed grep -i æ            0.30(0.96+0.39)
    7821.12: basic grep -i æ            0.27(0.92+0.44)
    7821.13: extended grep -i æ         0.28(0.90+0.46)
    7821.14: perl grep -i æ             0.28(0.74+0.49)

I'm planning to change how fixed-string searching happens. This test
gives a baseline for comparing performance before & after any such
change.

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>