In the next commit, we want to teach Git's test suite to optionally
output test results in JUnit-style .xml files. These files contain
information about the time spent. So we need a way to measure time.
While we could use `date +%s` for that, this will give us only seconds,
i.e. very coarse-grained timings.
GNU `date` supports `date +%s.%N` (i.e. nanosecond-precision output),
but there is no equivalent in BSD `date` (read: on macOS, we would not
be able to obtain precise timings).
So let's introduce `test-tool date getnanos`, with an optional start
time, that outputs preciser values.
Granted, it is a bit pointless to try measuring times accurately in
shell scripts, certainly to nanosecond precision. But it is better than
second-granularity.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
ee0383cf1e