"git diff(n)" without --base, --ours, etc. defaults to --cc,
which usually is the same as -p unless you are in the middle of
a conflicted merge, just like the shell script version.
"git diff(n) blobA blobB path" complains and dies.
"git diff(n) tree0 tree1 tree2...treeN" does combined diff that
shows a merge of tree1..treeN to result in tree0.
Giving "-c" option to any command that defaults to "--cc" turns
off dense-combined flag.
Signed-off-by: Junio C Hamano <junkio@cox.net>
