built-in "git grep"

built-in "git grep"

This attempts to set up built-in "git grep" to further reduce
our dependence on the shell, while at the same time optionally
allowing to run grep against object database.  You could do
funky things like these:

	git grep --cached -e pattern	;# grep from index
	git grep -e pattern master	;# or in a rev
	git grep -e pattern master next ;# or in multiple revs
	git grep -e pattern pu^@	;# even like this with an
					;# extension from another topic ;-)
	git grep -e pattern master..next ;# or even from rev ranges
	git grep -e pattern master~20:Documentation
					;# or an arbitrary tree
	git grep -e pattern next:git-commit.sh
        				;# or an arbitrary blob

Right now, it does not understand and/or obey many options grep
should accept, and the pattern must be given with -e option due
to the way the parameter parser is structured, both of which
obviously need to be fixed for usability.

But this is going in the right direction.  The shell script
version is one of the worst Portability offender in the git
barebone Porcelainish; it uses xargs -0 to pass paths around and
shell arrays to sift flags and parameters.

Signed-off-by: Junio C Hamano <junkio@cox.net>