grep: add -r/--[no-]recursive

grep: add -r/--[no-]recursive

Recognize -r and --recursive as synonyms for --max-depth=-1 for
compatibility with GNU grep; it's still the default for git grep.

This also adds --no-recursive as synonym for --max-depth=0 for free,
which is welcome for completeness and consistency.

Fix the description for --max-depth, while we're at it -- negative
values other than -1 actually disable recursion, i.e. they are
equivalent to --max-depth=0.

Requested-by: Christoph Berg <myon@debian.org>
Suggested-by: Junio C Hamano <gitster@pobox.com>
Initial-patch-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>