This adds '-e' option to git-cat-file, to test for the existence
of the object.

This also cleans up the option-parsing in git-cat-file slightly.

[jc: HPA version had -n option which did rev-parse --verify; the
real value of this patch is the option parsing cleanup.]
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>
Signed-off-by: NJunio C Hamano <junkio@cox.net>