run-command: show prepared command

This adds a trace point in start_command so we can see the full
command invocation without having to resort to strace/code inspection.
For example:

 $ GIT_TRACE=1 git test foo
 git.c:755               trace: exec: git-test foo
 run-command.c:657       trace: run_command: git-test foo
 run-command.c:657       trace: run_command: 'echo $*' foo
 run-command.c:749       trace: start_command: /bin/sh -c 'echo $* "$@"' 'echo $*' foo

Prior changes have made the documentation around the internals of the
alias command execution clearer, but I have still found this detailed
view of the aliased command being run helpful for debugging purposes.

A test case is added to ensure the full command output is present in
the execution flow.

Signed-off-by: Ian Wienand <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ianw

run-command.c

@@ -746,6 +746,8 @@ int start_command(struct child_process *cmd)
 		goto end_of_spawn;
 	}
 
+	trace_argv_printf(&argv.v[1], "trace: start_command:");
+
 	if (pipe(notify_pipe))
 		notify_pipe[0] = notify_pipe[1] = -1;
 
@@ -913,6 +915,7 @@ int start_command(struct child_process *cmd)
 	else if (cmd->use_shell)
 		cmd->args.v = prepare_shell_cmd(&nargv, sargv);
 
+	trace_argv_printf(cmd->args.v, "trace: start_command:");
 	cmd->pid = mingw_spawnvpe(cmd->args.v[0], cmd->args.v,
 				  (char**) cmd->env.v,
 				  cmd->dir, fhin, fhout, fherr);

t/t0014-alias.sh

@@ -44,4 +44,15 @@ test_expect_success 'run-command formats empty args properly' '
     test_cmp expect actual
 '
 
+test_expect_success 'tracing a shell alias with arguments shows trace of prepared command' '
+	cat >expect <<-EOF &&
+	trace: start_command: SHELL -c ${SQ}echo \$* "\$@"${SQ} ${SQ}echo \$*${SQ} arg
+	EOF
+	git config alias.echo "!echo \$*" &&
+	env GIT_TRACE=1 git echo arg 2>output &&
+	# redact platform differences
+	sed -n -e "s/^\(trace: start_command:\) .* -c /\1 SHELL -c /p" output >actual &&
+	test_cmp expect actual
+'
+
 test_done