sscanf/strtoul: parse integers robustly

sscanf/strtoul: parse integers robustly

* builtin-grep.c (strtoul_ui): Move function definition from here, to...
* git-compat-util.h (strtoul_ui): ...here, with an added "base" parameter.
* builtin-grep.c (cmd_grep): Update use of strtoul_ui to include base, "10".
* builtin-update-index.c (read_index_info): Diagnose an invalid mode integer
that is out of range or merely larger than INT_MAX.
(cmd_update_index): Use strtoul_ui, not sscanf.
* convert-objects.c (write_subdirectory): Likewise.

Signed-off-by: Jim Meyering <jim@meyering.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>