add, update-index: fix --chmod argument help

add, update-index: fix --chmod argument help

Don't translate the argument specification for --chmod; "+x" and "-x"
are the literal strings that the commands accept.

Separate alternatives using a pipe character instead of a slash, for
consistency.

Use the flag PARSE_OPT_LITERAL_ARGHELP to prevent parseopt from adding a
pair of angular brackets around the argument help string, as that would
wrongly indicate that users need to replace the literal strings with
some kind of value.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>