exclude-promisor-objects: declare when option is allowed

The --exclude-promisor-objects option causes some funny behavior in at
least two commands: log and blame. It causes a BUG crash:

	$ git log --exclude-promisor-objects
	BUG: revision.c:2143: exclude_promisor_objects can only be used
	when fetch_if_missing is 0
	Aborted
	[134]

Fix this such that the option is treated like any other unknown option.
The commands that must support it are limited, so declare in those
commands that the flag is supported. In particular:

	pack-objects
	prune
	rev-list

The commands were found by searching for logic which parses
--exclude-promisor-objects outside of revision.c. Extra logic outside of
revision.c is needed because fetch_if_missing must be turned on before
revision.c sees the option or it will BUG-crash. The above list is
supported by the fact that no other command is introspectively invoked
by another command passing --exclude-promisor-object.

Signed-off-by: Matthew DeVore <matvore@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -2853,6 +2853,7 @@ static void get_object_list(int ac, const char **av)
 
 	init_revisions(&revs, NULL);
 	save_commit_buffer = 0;
+	revs.allow_exclude_promisor_objects_opt = 1;
 	setup_revisions(ac, av, &revs, NULL);
 
 	/* make sure shallows are read */