Merge branch 'tg/grep-no-index-fallback'

"git grep" by default does not fall back to its "--no-index" behaviour outside a directory under Git's control (otherwise the user may by mistake end up running a huge recursive search); with a new configuration (set in $HOME/.gitconfig--by definition this cannot be set in the config file per project), this safety can be disabled. * tg/grep-no-index-fallback: builtin/grep: add grep.fallbackToNoIndex config t7810: correct --no-index test