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

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