[CI] Select clang-format files by command-line tools
[CI] Select clang-format files by command-line tools

Use command-line utilities to select the files which are checked by
clang-format.

Azure Pipelines hosted runners use Ubuntu 16.04, which uses a dated
version of git which does not support to use `pathspec` to exclude
certain files/folders from the diff output.

Fixes #227

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan