builtin/grep: add grep.fallbackToNoIndex config

builtin/grep: add grep.fallbackToNoIndex config

Currently when git grep is used outside of a git repository without the
--no-index option git simply dies.  For convenience, add a
grep.fallbackToNoIndex configuration variable.  If set to true, git grep
behaves like git grep --no-index if it is run outside of a git
repository.  It defaults to false, preserving the current behavior.

Helped-by: Jeff King <peff@peff.net>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>