cache-tree: fix strbuf growth in prime_cache_tree_rec()

Use size_t to store the original length of the strbuf tree_len, as
that's the correct type.

Don't double the allocated size of the strbuf when adding a subdirectory
name.  And the chance of the trailing slash fitting in the slack left by
strbuf_add() is very high, so stop pre-growing the strbuf at all.

Suggested-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: rscharfe

cache-tree.c

@@ -760,7 +760,7 @@ static void prime_cache_tree_rec(struct repository *r,
 	struct tree_desc desc;
 	struct name_entry entry;
 	int cnt;
-	int base_path_len = tree_path->len;
+	size_t base_path_len = tree_path->len;
 
 	oidcpy(&it->oid, &tree->object.oid);
 
@@ -785,7 +785,6 @@ static void prime_cache_tree_rec(struct repository *r,
 			 */
 			if (r->index->sparse_index) {
 				strbuf_setlen(tree_path, base_path_len);
-				strbuf_grow(tree_path, base_path_len + entry.pathlen + 1);
 				strbuf_add(tree_path, entry.path, entry.pathlen);
 				strbuf_addch(tree_path, '/');
 			}