pretty: switch hard-coded constants to the_hash_algo

Switch several hard-coded constants into expressions based either on
GIT_MAX_HEXSZ or the_hash_algo.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pretty.c

@@ -1575,7 +1575,7 @@ static void pp_header(struct pretty_print_context *pp,
 		}
 
 		if (starts_with(line, "parent ")) {
-			if (linelen != 48)
+			if (linelen != the_hash_algo->hexsz + 8)
 				die("bad parent line in commit");
 			continue;
 		}
@@ -1583,7 +1583,7 @@ static void pp_header(struct pretty_print_context *pp,
 		if (!parents_shown) {
 			unsigned num = commit_list_count(commit->parents);
 			/* with enough slop */
-			strbuf_grow(sb, num * 50 + 20);
+			strbuf_grow(sb, num * (GIT_MAX_HEXSZ + 10) + 20);
 			add_merge_info(pp, sb, commit);
 			parents_shown = 1;
 		}