Don't clang-format non checked out things

mbed-os is a directory that gets checked out by the mbed cli
(a tool similar in spirit to miot) and it contains a lot of
.c sources, so many of them that clang-format burns your CPU.

While we're at it, also fix check_no_ifdefs when running under a commit
hook.

PUBLISHED_FROM=e5dd457ea3c4f9bd880f2491fb322dffa7abf70a