hooks: allow input from stdin

Let hooks receive user input if applicable.

Closing stdin originates in f5bbc32 (Port git commit to C,
2007). Looks like the original shell implementation did have
stdin open. Not clear why the author chose to close it on
the C port (maybe copy&paste).

Some references of users requesting this feature. Some of
them use acrobatics to gain access to stdin:
[1] https://stackoverflow.com/q/1067874/764870
[2] https://stackoverflow.com/q/47477766/764870
[3] https://stackoverflow.com/q/3417896/764870
[4] PHP-CS-Fixer/PHP-CS-Fixer#3165
[5] typicode/husky#442

run-command.c

@@ -1356,7 +1356,6 @@ int run_hook_ve(const char *const *env, const char *name, va_list args)
 	while ((p = va_arg(args, const char *)))
 		strvec_push(&hook.args, p);
 	hook.env = env;
-	hook.no_stdin = 1;
 	hook.stdout_to_stderr = 1;
 	hook.trace2_hook_name = name;