worktree: fix calloc of the wrong object type

Author: tiennou

src/worktree.c

@@ -131,7 +131,7 @@ static int open_worktree_dir(git_worktree **out, const char *parent, const char
 		goto out;
 	}
 
-	if ((wt = git__calloc(1, sizeof(struct git_repository))) == NULL) {
+	if ((wt = git__calloc(1, sizeof(*wt))) == NULL) {
 		error = -1;
 		goto out;
 	}