Add "has_sha1_pack()" function to query whether the object is available in a pack

We'll want this for incremental packing.

cache.h

@@ -176,6 +176,7 @@ extern int read_tree(void *buffer, unsigned long size, int stage);
 
 extern int write_sha1_from_fd(const unsigned char *sha1, int fd);
 
+extern int has_sha1_pack(const unsigned char *sha1);
 extern int has_sha1_file(const unsigned char *sha1);
 
 /* Convert to/from hex/sha1 representation */

sha1_file.c

@@ -1267,6 +1267,12 @@ int write_sha1_from_fd(const unsigned char *sha1, int fd)
 	return 0;
 }
 
+int has_sha1_pack(const unsigned char *sha1)
+{
+	struct pack_entry e;
+	return find_pack_entry(sha1, &e);
+}
+
 int has_sha1_file(const unsigned char *sha1)
 {
 	struct stat st;