Extended SHA1 -- "rev^@" syntax to mean "all parents"

Extended SHA1 -- "rev^@" syntax to mean "all parents"

A short-hand "rev^@" is understood to be "all parents of the
named commit" with this patch.  So you can do

	git show v1.0.0^@

to view the parents of a merge commit,

	gitk ^v1.0.0^@ v1.0.4

to view the log between two revs (including the bottom one), and

	git diff --cc v1.1.0 v1.0.0^@

to inspect what got changed from the merge parents of v1.0.0 to v1.1.0.

This might be just my shiny new toy that is not very useful in
practice.  I needed it to do the multi-tree diff on Len's
infamous 12-way Octopus; typing "diff --cc funmerge funmerge^1
funmerge^2 funmerge^3 ..." was too painful.

[jc: taking suggestions from Linus and Johannes to match expectations
from shell users who are used to see $@ or $* either of which makes
sense.  I tend to write "$@" more often so...]

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