Make 'git diff --cached' synonymous to 'git diff --cached HEAD'.

Make 'git diff --cached' synonymous to 'git diff --cached HEAD'.

When making changes to different files (i.e. dirty working tree) and
committing logically separate changes in groups, often it is necessary
to run 'git diff --cached HEAD' to make sure that the changes being
committed makes sense.  Saying 'git diff --cached' by mistake gives
rather uninformative error message from git-diff-files complaining it
does not understand --cached flag.

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