diff against master (no origin), print needed formatting change

- also, don't run any clang-format if no c++ files were found
1 job for !960 with clang-format-gitlab-ci in 1 minute and 5 seconds (queued for 9 seconds)
Status Name Job ID Coverage
failed clang_format #118464


Name Stage Failure
clang_format Build

2021-07-17 04:32:47 (57.9 MB/s) - '.clang-format' saved [2547/2547]

$ git diff-tree --no-commit-id --name-only -r $CI_MERGE_REQUEST_TARGET_BRANCH_NAME -r $CI_COMMIT_SHA | grep -P '[.](c|cc|cpp|hpp|h)$' | xargs --no-run-if-empty --verbose clang-format-11 --dry-run -Werror
fatal: ambiguous argument 'master': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
Cleaning up file based variables
ERROR: Job failed: exit code 1