git-grep: clarification on parameters.

git-grep: clarification on parameters.

We forgot to make sure that there is no more than one pattern
parameter.  Also when looking for files in a directory called
'--others', it passed that path limiter without preceding the
end-of-options marker '--' to underlying git-ls-files, which
misunderstood it as one of its options instead.

	$ git grep --others -e Meta/Make Meta
	$ git grep -o -e Meta/Make Meta
	$ git grep -o Meta/Make Meta

look for a string "Meta/Make" from untracked files in Meta/
directory.

	$ git grep Meta/Make --others

looks for the same string from tracked files in ./--others
directory.

On the other hand,

	$ git grep -e Meta/Make --others

does not have a freestanding pattern, so everybody is parameter
and there is no path specifier.  It looks for the string in all
the untracked files without any path limiter.

[jc: updated with usability enhancements and documentation
cleanups from Sean.]

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