grep docs: de-duplicate configuration sections

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>