tests: make test_might_fail fail on missing commands

Detect and report hard-to-notice spelling mistakes like

 test_might_fail "git config --unset whatever"

(the extra quotes prevent the shell from running git as intended;
instead, the shell looks for a "git config --unset whatever" file).

Cc: Jeff King <peff@peff.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -622,6 +622,9 @@ test_might_fail () {
 	if test $exit_code -gt 129 -a $exit_code -le 192; then
 		echo >&2 "test_might_fail: died by signal: $*"
 		return 1
+	elif test $exit_code = 127; then
+		echo >&2 "test_might_fail: command not found: $*"
+		return 1
 	fi
 	return 0
 }