[PATCH] Add "git grep" helper

[PATCH] Add "git grep" helper

Very convenient shorthand for

	git-ls-files [file-patterns] | xargs grep <pattern>

which I tend to do all the time.

Yes, it's trivial, but it's really nice. I can do

	git grep '\<some_variable\>' arch/i386 include/asm-i386

and it does exactly what you'd think it does. And since it just uses the
normal git-ls-files file patterns, you can do things like

	git grep something 'include/*.h'

and it will search all header files under the include/ subdirectory.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>