sub-process: do not borrow cmd pointer from caller

Teach subprocess_start() to use a copy of the passed `cmd` string
rather than borrowing the buffer from the caller.

Some callers of subprocess_start() pass the value returned from
find_hook() which points to a static buffer and therefore is only
good until the next call to find_hook().  This could cause problems
for the long-running background processes managed by sub-process.c
where later calls to subprocess_find_entry() to get an existing
process will fail.  This could cause more than 1 long-running
process to be created.

TODO Need to confirm, but if only read_object_hook() uses
TODO subprocess_start() in this manner, we could drop this
TODO commit when we drop support for read_object_hook().

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>

sub-process.c

@@ -81,7 +81,12 @@ int subprocess_start(struct hashmap *hashmap, struct subprocess_entry *entry, co
 	int err;
 	struct child_process *process;
 
-	entry->cmd = cmd;
+	// BUGBUG most callers to subprocess_start() pass in "cmd" the value
+	// BUGBUG of find_hook() which returns a static buffer (that's only
+	// BUGBUG good until the next call to find_hook()).
+	// BUGFIX Defer assignment until we copy the string in our argv.
+	// entry->cmd = cmd;
+
 	process = &entry->process;
 
 	child_process_init(process);
@@ -93,6 +98,8 @@ int subprocess_start(struct hashmap *hashmap, struct subprocess_entry *entry, co
 	process->clean_on_exit_handler = subprocess_exit_handler;
 	process->trace2_child_class = "subprocess";
 
+	entry->cmd = process->args.v[0];
+
 	err = start_command(process);
 	if (err) {
 		error("cannot fork to run subprocess '%s'", cmd);