pack-bitmap.c: introduce 'bitmap_is_preferred_refname()'

In a recent commit, pack-objects learned support for the
'pack.preferBitmapTips' configuration. This patch prepares the
multi-pack bitmap code to respect this configuration, too.

The yet-to-be implemented code will find that it is more efficient to
check whether each reference contains a prefix found in the configured
set of values rather than doing an additional traversal.

Implement a function 'bitmap_is_preferred_refname()' which will perform
that check. Its caller will be added in a subsequent patch.

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

pack-bitmap.c

@@ -1658,3 +1658,19 @@ const struct string_list *bitmap_preferred_tips(struct repository *r)
 {
 	return repo_config_get_value_multi(r, "pack.preferbitmaptips");
 }
+
+int bitmap_is_preferred_refname(struct repository *r, const char *refname)
+{
+	const struct string_list *preferred_tips = bitmap_preferred_tips(r);
+	struct string_list_item *item;
+
+	if (!preferred_tips)
+		return 0;
+
+	for_each_string_list_item(item, preferred_tips) {
+		if (starts_with(refname, item->string))
+			return 1;
+	}
+
+	return 0;
+}

pack-bitmap.h

@@ -94,5 +94,6 @@ void bitmap_writer_finish(struct pack_idx_entry **index,
 			  uint16_t options);
 
 const struct string_list *bitmap_preferred_tips(struct repository *r);
+int bitmap_is_preferred_refname(struct repository *r, const char *refname);
 
 #endif