pack-objects: factor out handling '--stdin-packs'

At the bottom of cmd_pack_objects() we check which mode the command is
running in (e.g., generating a cruft pack, handling '--stdin-packs',
using the internal rev-list, etc.) and handle the mode appropriately.

The '--stdin-packs' case is handled inline (dating back to its
introduction in 339bce2 (builtin/pack-objects.c: add '--stdin-packs'
option, 2021-02-22)) since it is relatively short. Extract the body of
"if (stdin_packs)" into its own function to prepare for the
implementation to become lengthier in a following commit.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: ttaylorr

builtin/pack-objects.c

@@ -3909,6 +3909,17 @@ static void read_packs_list_from_stdin(void)
 	string_list_clear(&exclude_packs, 0);
 }
 
+static void add_unreachable_loose_objects(void);
+
+static void read_stdin_packs(int rev_list_unpacked)
+{
+	/* avoids adding objects in excluded packs */
+	ignore_packed_keep_in_core = 1;
+	read_packs_list_from_stdin();
+	if (rev_list_unpacked)
+		add_unreachable_loose_objects();
+}
+
 static void add_cruft_object_entry(const struct object_id *oid, enum object_type type,
 				   struct packed_git *pack, off_t offset,
 				   const char *name, uint32_t mtime)
@@ -4004,7 +4015,6 @@ static void mark_pack_kept_in_core(struct string_list *packs, unsigned keep)
 	}
 }
 
-static void add_unreachable_loose_objects(void);
 static void add_objects_in_unpacked_packs(void);
 
 static void enumerate_cruft_objects(void)
@@ -5135,11 +5145,7 @@ int cmd_pack_objects(int argc,
 		progress_state = start_progress(the_repository,
 						_("Enumerating objects"), 0);
 	if (stdin_packs) {
-		/* avoids adding objects in excluded packs */
-		ignore_packed_keep_in_core = 1;
-		read_packs_list_from_stdin();
-		if (rev_list_unpacked)
-			add_unreachable_loose_objects();
+		read_stdin_packs(rev_list_unpacked);
 	} else if (cruft) {
 		read_cruft_objects();
 	} else if (!use_internal_rev_list) {