We use `git__strtol64` to parse file modes of the index entries, which
does not limit the parsed buffer length. As the index can be essentially
treated as "untrusted" in that the data stems from the file system, it
may be misformatted and may not contain terminating `NUL` bytes. This
may lead to out-of-bounds reads when trying to parse index entries with
such malformatted modes.

Fix the issue by using `git__strntol64` instead.