fsmonitor: use internal argv_array of struct child_process

fsmonitor: use internal argv_array of struct child_process

Avoid magic array sizes and indexes by constructing the fsmonitor
command line using the embedded argv_array of the child_process.  The
resulting code is shorter and easier to extend.

Getting rid of the snprintf() calls is a bonus -- even though the
buffers were big enough here to avoid truncation -- as it makes auditing
the remaining callers easier.

Inspired-by: Jeff King <redacted>
Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>