avoid file-or-branch ambiguity by adding -- to `git` command

1 job for !960 with clang-format-gitlab-ci in 1 minute and 7 seconds (queued for 4 seconds)
detached
Status Name Job ID Coverage
  Build
failed clang_format #118465

00:01:07

 
Name Stage Failure
failed
clang_format Build

0K .. 100% 57.5M=0s

2021-07-17 04:56:40 (57.5 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: bad revision 'master'
Cleaning up file based variables
ERROR: Job failed: exit code 1