hooks: Add a first-approximation clang-format hook

This change adds a small wrapper that calls git-clang-format and prints
out any files that need attention.

Bug: 26800693
Test: Added clang_format builtin hook to a project, complained about
      non-compliant files.

Change-Id: Id4431aaed4d17f6639a448fbddf1a595168cf27f