bash completion: Add completion for 'git grep'

bash completion: Add completion for 'git grep'

Add completions for all long options specified in the docs

    --cached
    --text --ignore-case --word-regexp --invert-match
    --full-name
    --extended-regexp --basic-regexp --fixed-strings
    --files-with-matches --name-only
    --files-without-match
    --count
    --and --or --not --all-match

Signed-off-by: Lee Marlow <lee.marlow@gmail.com>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>