add -i: add extra options at the right place in "diff" command line

Appending "--diff-algorithm=histogram" at the end of canned command
line for various modes of "diff" is correct for most of them but not
for "stash" that has a non-option already wired in, like so:

	'stash' => {
		DIFF => 'diff-index -p HEAD',

Appending an extra option after non-option may happen to work due to
overly lax command line parser, but that is not something we should
rely on.  Instead, splice in the extra argument immediately after the
command name (i.e. 'diff-index', 'diff-files', etc.).

Signed-off-by: Junio C Hamano <gitster@pobox.com>