grep docs: de-duplicate configuration sections

Include the "config/grep.txt" file in "git-grep.txt", instead of repeating an almost identical description of the "grep" configuration variables in two places. There is no loss of information here that isn't shown in the addition to "grep.txt". This change was made by copying the contents of "git-grep.txt"'s version over the "grep.txt" version. Aside from the change "grep.txt" being made here the two were identical. This documentation started being copy/pasted around in b22520a37c8 (grep: allow -E and -n to be turned on by default via configuration, 2011-03-30). After that in e.g. 6453f7b3486 (grep: add grep.fullName config variable, 2014-03-17) they started drifting apart, with only grep.fullName being described in the command documentation. In 434e6e753fe (config.txt: move grep.* to a separate file, 2018-10-27) we gained the include, but didn't do this next step, let's do it now. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Reviewed-by: Matheus Tavares <matheus.bernardino@usp.br> Signed-off-by: Junio C Hamano <gitster@pobox.com>