t/perf/fsmonitor: add benchmark for dirty status

t/perf/fsmonitor: add benchmark for dirty status

This benchmark covers the git status time for a heavily
dirty directory - benchmarking fsmonitor's refresh

When running to compare our perl vs rs-git-fsmonitor - we see that
the perl script incurs significant overhead - further motivation
to provide a faster implementation within git.

7519.7: status (dirty) (fsmonitor=query-watchman) 10.05(7.78+1.56)
7519.20: status (dirty) (fsmonitor=rs-git-fsmonitor) 6.72(4.37+1.64)
7519.33: status (dirty) (fsmonitor=disabled) 5.62(4.24+2.03)

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