'git diff-index' doesn't honor the 'diff.algorithm' variable

Hi all,

Subj. ...even though it's explicitly mentioned in the subcommand's man 
page. Git version 2.7.4 here.

To elaborate:

- Call 'git config --global diff.algorithm histogram'.

- Try the example from http://stackoverflow.com/a/36551123/615245.

'git diff test.css' gives the expected output using the non-default 
algorithm.

'git diff-index -p HEAD -- test.css' uses the default one.

Best regards,
Dmitry.