Fix "git-diff-script A B"

When "git-diff-script A..B" notation was introduced, it ended up breaking
the traditional two revisions notation.

[jc: there are other issues with the current "git diff" I would like to
 address, but they would be left to later rounds.  For example, -M and -p flags
 should not be hardcoded default, and it shouldn't be too hard to rewrite
 the script without using shell arrays.]

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