t/perf/p7519-fsmonitor.sh: warm cache on first git status

t/perf/p7519-fsmonitor.sh: warm cache on first git status

The first git status would be inflated due to warming of
filesystem cache. This makes the results comparable.

Before
Test                                                             this tree
--------------------------------------------------------------------------------
7519.2: status (fsmonitor=.git/hooks/fsmonitor-watchman)         2.52(1.59+1.56)
7519.3: status -uno (fsmonitor=.git/hooks/fsmonitor-watchman)    0.18(0.12+0.06)
7519.4: status -uall (fsmonitor=.git/hooks/fsmonitor-watchman)   1.36(0.73+0.62)
7519.7: status (fsmonitor=)                                      0.69(0.52+0.90)
7519.8: status -uno (fsmonitor=)                                 0.37(0.28+0.81)
7519.9: status -uall (fsmonitor=)                                1.53(0.93+1.32)

After
Test                                                             this tree
--------------------------------------------------------------------------------
7519.2: status (fsmonitor=.git/hooks/fsmonitor-watchman)         0.39(0.33+0.06)
7519.3: status -uno (fsmonitor=.git/hooks/fsmonitor-watchman)    0.17(0.13+0.05)
7519.4: status -uall (fsmonitor=.git/hooks/fsmonitor-watchman)   1.34(0.77+0.56)
7519.7: status (fsmonitor=)                                      0.70(0.53+0.90)
7519.8: status -uno (fsmonitor=)                                 0.37(0.32+0.78)
7519.9: status -uall (fsmonitor=)                                1.55(1.01+1.25)

Signed-off-by: Nipunn Koorapati <redacted>
Signed-off-by: Junio C Hamano <redacted>