fsmonitor: add perf test for git diff HEAD

fsmonitor: add perf test for git diff HEAD

Update the xargs call so that if your large repo contains
symlinks, test-tool chmtime failure does not end the script.

On Linux
Test                                                          this tree           upstream/master
---------------------------------------------------------------------------------------------------------
7519.4: status (fsmonitor=fsmonitor-watchman)                 0.52(0.43+0.10)     0.53(0.49+0.05) +1.9%
7519.5: status -uno (fsmonitor=fsmonitor-watchman)            0.21(0.15+0.07)     0.22(0.13+0.09) +4.8%
7519.6: status -uall (fsmonitor=fsmonitor-watchman)           1.65(0.93+0.71)     1.69(1.03+0.65) +2.4%
7519.7: status (dirty) (fsmonitor=fsmonitor-watchman)         11.99(11.34+1.58)   11.95(11.02+1.79) -0.3%
7519.8: diff (fsmonitor=fsmonitor-watchman)                   0.25(0.17+0.26)     0.25(0.18+0.26) +0.0%
7519.9: diff HEAD (fsmonitor=fsmonitor-watchman)              0.39(0.25+0.34)     0.89(0.35+0.74) +128.2%
7519.10: diff -- 0_files (fsmonitor=fsmonitor-watchman)       0.16(0.13+0.04)     0.16(0.12+0.05) +0.0%
7519.11: diff -- 10_files (fsmonitor=fsmonitor-watchman)      0.16(0.12+0.05)     0.16(0.12+0.05) +0.0%
7519.12: diff -- 100_files (fsmonitor=fsmonitor-watchman)     0.16(0.12+0.05)     0.16(0.12+0.05) +0.0%
7519.13: diff -- 1000_files (fsmonitor=fsmonitor-watchman)    0.16(0.11+0.06)     0.16(0.12+0.05) +0.0%
7519.14: diff -- 10000_files (fsmonitor=fsmonitor-watchman)   0.18(0.13+0.06)     0.17(0.10+0.08) -5.6%
7519.15: add (fsmonitor=fsmonitor-watchman)                   2.25(1.53+0.68)     2.25(1.47+0.74) +0.0%
7519.18: status (fsmonitor=disabled)                          0.88(0.73+1.03)     0.89(0.67+1.08) +1.1%
7519.19: status -uno (fsmonitor=disabled)                     0.45(0.43+0.89)     0.45(0.34+0.98) +0.0%
7519.20: status -uall (fsmonitor=disabled)                    1.88(1.16+1.58)     1.88(1.22+1.51) +0.0%
7519.21: status (dirty) (fsmonitor=disabled)                  7.53(7.05+2.11)     7.53(6.98+2.04) +0.0%
7519.22: diff (fsmonitor=disabled)                            0.42(0.37+0.92)     0.42(0.38+0.91) +0.0%
7519.23: diff HEAD (fsmonitor=disabled)                       0.44(0.41+0.90)     0.44(0.40+0.91) +0.0%
7519.24: diff -- 0_files (fsmonitor=disabled)                 0.13(0.09+0.05)     0.13(0.09+0.05) +0.0%
7519.25: diff -- 10_files (fsmonitor=disabled)                0.13(0.10+0.04)     0.13(0.10+0.04) +0.0%
7519.26: diff -- 100_files (fsmonitor=disabled)               0.13(0.09+0.05)     0.13(0.10+0.04) +0.0%
7519.27: diff -- 1000_files (fsmonitor=disabled)              0.13(0.09+0.06)     0.13(0.09+0.05) +0.0%
7519.28: diff -- 10000_files (fsmonitor=disabled)             0.14(0.11+0.05)     0.14(0.10+0.05) +0.0%
7519.29: add (fsmonitor=disabled)                             2.43(1.61+1.64)     2.43(1.69+1.57) +0.0%

On linux (2.29.2 vs w/ this patch):
nipunn@nipunn-dbx:~/src/server3$ strace -f -c git diff 2>&1 | grep lstat
  0.04    0.000063           3        20         6 lstat
nipunn@nipunn-dbx:~/src/server3$ strace -f -c git diff HEAD 2>&1 | grep lstat
 94.98    5.242262          10    523783        13 lstat
nipunn@nipunn-dbx:~/src/server3$ strace -f -c ../git/bin-wrappers/git diff 2>&1 | grep lstat
  0.38    0.000032           5         7         3 lstat
nipunn@nipunn-dbx:~/src/server3$ strace -f -c ../git/bin-wrappers/git diff HEAD 2>&1 | grep lstat
 99.44    0.741892           9     81634        10 lstat

On mac (2.29.2 vs w/ this patch):
nipunn-mbp:server nipunn$ sudo dtruss -L -f -c git diff 2>&1 | grep "^lstat64 "
lstat64                                         8
nipunn-mbp:server nipunn$ sudo dtruss -L -f -c git diff HEAD 2>&1 | grep "^lstat64 "
lstat64                                    120242
nipunn-mbp:server nipunn$ sudo dtruss -L -f -c ../git/bin-wrappers/git diff 2>&1 | grep "^lstat64 "
lstat64                                         4
nipunn-mbp:server nipunn$ sudo dtruss -L -f -c ../git/bin-wrappers/git diff HEAD 2>&1 | grep "^lstat64 "
lstat64                                      4497

There are still a bunch of lstats - on directories, but not every file. Progress!

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