tests: introduce --stress-jobs=<N>

The --stress option currently accepts an argument, but it is confusing
to at least this user that the argument does not define the maximal
number of stress iterations, but instead the number of jobs to run in
parallel per stress iteration.

Let's introduce a separate option for that, whose name makes it more
obvious what it is about, and let --stress=<N> error out with a helpful
suggestion about the two options tha could possibly have been meant.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/README

@@ -196,11 +196,10 @@ appropriately before running "make".
 	variable to "1" or "0", respectively.
 
 --stress::
---stress=<N>::
 	Run the test script repeatedly in multiple parallel jobs until
 	one of them fails.  Useful for reproducing rare failures in
 	flaky tests.  The number of parallel jobs is, in order of
-	precedence: <N>, or the value of the GIT_TEST_STRESS_LOAD
+	precedence: the value of the GIT_TEST_STRESS_LOAD
 	environment variable, or twice the number of available
 	processors (as shown by the 'getconf' utility),	or 8.
 	Implies `--verbose -x --immediate` to get the most information
@@ -211,6 +210,9 @@ appropriately before running "make".
 	'.stress-<nr>' suffix, and the trash directory of the failed
 	test job is renamed to end with a '.stress-failed' suffix.
 
+--stress-jobs=<N>::
+	Override the number of parallel jobs. Implies `--stress`.
+
 --stress-limit=<N>::
 	When combined with --stress run the test script repeatedly
 	this many times in each of the parallel jobs or until one of

t/test-lib.sh

@@ -147,10 +147,16 @@ do
 	--stress)
 		stress=t ;;
 	--stress=*)
+		echo "error: --stress does not accept an argument: '$opt'" >&2
+		echo "did you mean --stress-jobs=${opt#*=} or --stress-limit=${opt#*=}?" >&2
+		exit 1
+		;;
+	--stress-jobs=*)
+		stress=t;
 		stress=${opt#--*=}
 		case "$stress" in
 		*[!0-9]*|0*|"")
-			echo "error: --stress=<N> requires the number of jobs to run" >&2
+			echo "error: --stress-jobs=<N> requires the number of jobs to run" >&2
 			exit 1
 			;;
 		*)	# Good.