built-in add -p: handle diff.algorithm

built-in add -p: handle diff.algorithm

The Perl version of `git add -p` reads the config setting
`diff.algorithm` and if set, uses it to generate the diff using the
specified algorithm.

This patch ports that functionality to the C version.

Note: just like `git-add--interactive.perl`, we do _not_ respect this
config setting in `git add -i`'s `diff` command, but _only_ in the
`patch` command.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>