start_command: detect execvp failures early

Previously, failures during execvp could be detected only by
finish_command. However, in some situations it is beneficial for the
parent process to know earlier that the child process will not run.

The idea to use a pipe to signal failures to the parent process and
the test case were lifted from patches by Ilari Liusvaara.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t0061-run-command.sh b/t/t0061-run-command.sh
new file mode 100755
index 0000000..10b26e4
--- /dev/null
+++ b/t/t0061-run-command.sh
@@ -0,0 +1,14 @@
+#!/bin/sh
+#
+# Copyright (c) 2009 Ilari Liusvaara
+#
+
+test_description='Test run command'
+
+. ./test-lib.sh
+
+test_expect_success 'start_command reports ENOENT' '
+	test-run-command start-command-ENOENT ./does-not-exist
+'
+
+test_done