test-date: add a subcommand to measure times in shell scripts

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. Note that this might not actually
give us nanosecond precision on some platforms, but it will give us as
precise information as possible, without the portability issues of shell
commands.

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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-date.c

@@ -7,6 +7,7 @@ static const char *usage_msg = "\n"
 "  test-tool date parse [date]...\n"
 "  test-tool date approxidate [date]...\n"
 "  test-tool date timestamp [date]...\n"
+"  test-tool date getnanos [start-nanos]\n"
 "  test-tool date is64bit\n"
 "  test-tool date time_t-is64bit\n";
 
@@ -82,6 +83,15 @@ static void parse_approx_timestamp(const char **argv, struct timeval *now)
 	}
 }
 
+static void getnanos(const char **argv, struct timeval *now)
+{
+	double seconds = getnanotime() / 1.0e9;
+
+	if (*argv)
+		seconds -= strtod(*argv, NULL);
+	printf("%lf\n", seconds);
+}
+
 int cmd__date(int argc, const char **argv)
 {
 	struct timeval now;
@@ -108,6 +118,8 @@ int cmd__date(int argc, const char **argv)
 		parse_approxidate(argv+1, &now);
 	else if (!strcmp(*argv, "timestamp"))
 		parse_approx_timestamp(argv+1, &now);
+	else if (!strcmp(*argv, "getnanos"))
+		getnanos(argv+1, &now);
 	else if (!strcmp(*argv, "is64bit"))
 		return sizeof(timestamp_t) == 8 ? 0 : 1;
 	else if (!strcmp(*argv, "time_t-is64bit"))