t/perf: add fsmonitor perf test for git diff

t/perf: add fsmonitor perf test for git diff

Results for the git-diff fsmonitor optimization
in patch in the parent-rev (using a 400k file repo to test)

As you can see here - git diff with fsmonitor running is
significantly better with this patch series (80% faster on my
workload)!

GIT_PERF_LARGE_REPO=~/src/server ./run v2.29.0-rc1 . -- p7519-fsmonitor.sh

Test                                                                     v2.29.0-rc1       this tree
-----------------------------------------------------------------------------------------------------------------
7519.2: status (fsmonitor=.git/hooks/fsmonitor-watchman)                 1.46(0.82+0.64)   1.47(0.83+0.62) +0.7%
7519.3: status -uno (fsmonitor=.git/hooks/fsmonitor-watchman)            0.16(0.12+0.04)   0.17(0.12+0.05) +6.3%
7519.4: status -uall (fsmonitor=.git/hooks/fsmonitor-watchman)           1.36(0.73+0.62)   1.37(0.76+0.60) +0.7%
7519.5: diff (fsmonitor=.git/hooks/fsmonitor-watchman)                   0.85(0.22+0.63)   0.14(0.10+0.05) -83.5%
7519.6: diff -- 0_files (fsmonitor=.git/hooks/fsmonitor-watchman)        0.12(0.08+0.05)   0.13(0.11+0.02) +8.3%
7519.7: diff -- 10_files (fsmonitor=.git/hooks/fsmonitor-watchman)       0.12(0.08+0.04)   0.13(0.09+0.04) +8.3%
7519.8: diff -- 100_files (fsmonitor=.git/hooks/fsmonitor-watchman)      0.12(0.07+0.05)   0.13(0.07+0.06) +8.3%
7519.9: diff -- 1000_files (fsmonitor=.git/hooks/fsmonitor-watchman)     0.12(0.09+0.04)   0.13(0.08+0.05) +8.3%
7519.10: diff -- 10000_files (fsmonitor=.git/hooks/fsmonitor-watchman)   0.14(0.09+0.05)   0.13(0.10+0.03) -7.1%
7519.12: status (fsmonitor=)                                             1.67(0.93+1.49)   1.67(0.99+1.42) +0.0%
7519.13: status -uno (fsmonitor=)                                        0.37(0.30+0.82)   0.37(0.33+0.79) +0.0%
7519.14: status -uall (fsmonitor=)                                       1.58(0.97+1.35)   1.57(0.86+1.45) -0.6%
7519.15: diff (fsmonitor=)                                               0.34(0.28+0.83)   0.34(0.27+0.83) +0.0%
7519.16: diff -- 0_files (fsmonitor=)                                    0.09(0.06+0.04)   0.09(0.08+0.02) +0.0%
7519.17: diff -- 10_files (fsmonitor=)                                   0.09(0.07+0.03)   0.09(0.06+0.05) +0.0%
7519.18: diff -- 100_files (fsmonitor=)                                  0.09(0.06+0.04)   0.09(0.06+0.04) +0.0%
7519.19: diff -- 1000_files (fsmonitor=)                                 0.09(0.06+0.04)   0.09(0.05+0.05) +0.0%
7519.20: diff -- 10000_files (fsmonitor=)                                0.10(0.08+0.04)   0.10(0.06+0.05) +0.0%

I also added a benchmark for a tiny git diff workload w/ a pathspec.
I see an approximately .02 second overhead added w/ and w/o fsmonitor

From looking at these results, I suspected that refresh_fsmonitor
is already happening during git diff - independent of this patch
series' optimization. Confirmed that suspicion by breaking on
refresh_fsmonitor.

(gdb) bt  [simplified]
0  refresh_fsmonitor  at fsmonitor.c:176
1  ie_match_stat  at read-cache.c:375
2  match_stat_with_submodule at diff-lib.c:237
4  builtin_diff_files  at builtin/diff.c:260
5  cmd_diff  at builtin/diff.c:541
6  run_builtin  at git.c:450
7  handle_builtin  at git.c:700
8  run_argv  at git.c:767
9  cmd_main  at git.c:898
10 main  at common-main.c:52

Signed-off-by: Nipunn Koorapati <nipunn@dropbox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>