devel: provide external git diff driver for *.abi files [ci skip]

This adds an external diff driver for *.abi files, that shows only
interesting changes in those files.  This would be useful when adding
a new API.

Signed-off-by: Daiki Ueno <[email protected]>
Status Job ID Name Coverage
  External
passed Codacy/PR Quality Review

passed LGTM analysis: C/C++

00:05:52

passed LGTM analysis: JavaScript

00:06:00