help: fix leaking return value from help_unknown_cmd()

While `help_unknown_cmd()` would usually die on an unknown command, it
instead returns an autocorrected command when "help.autocorrect" is set.
But while the function is declared to return a string constant, it
actually returns an allocated string in that case. Callers thus aren't
aware that they have to free the string, leading to a memory leak.

Fix the function return type to be non-constant and free the returned
value at its only callsite.

Note that we cannot simply take ownership of `main_cmds.names[0]->name`
and then eventually free it. This is because the `struct cmdname` is
using a flex array to allocate the name, so the name pointer points into
the middle of the structure and thus cannot be freed.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git.c

@@ -961,7 +961,9 @@ int cmd_main(int argc, const char **argv)
 			exit(1);
 		}
 		if (!done_help) {
-			strvec_replace(&args, 0, help_unknown_cmd(cmd));
+			char *assumed = help_unknown_cmd(cmd);
+			strvec_replace(&args, 0, assumed);
+			free(assumed);
 			cmd = args.v[0];
 			done_help = 1;
 		} else {