grep: teach --untracked and --exclude-standard options

grep: teach --untracked and --exclude-standard options

In a working tree of a git managed repository, "grep --untracked" would
find the specified patterns from files in untracked files in addition to
its usual behaviour of finding them in the tracked files.

By default, when working with "--no-index" option, "grep" does not pay
attention to .gitignore mechanism. "grep --no-index --exclude-standard"
can be used to tell the command to use .gitignore and stop reporting hits
from files that would be ignored. Also, when working without "--no-index",
"grep" honors .gitignore mechanism, and "grep --no-exclude-standard" can
be used to tell the command to include hits from files that are ignored.

Signed-off-by: Junio C Hamano <gitster@pobox.com>