This patch ensures that the CI script controls which file extensions are
considered instead of letting git-clang-format apply its own filtering
rules. In particular, this properly handles libc++ extension-less
headers which were passed to git-clang-format, but then dropped by
the tool as having an unrecognized extension.
This is a second attempt to land 7620fe0d2d1e, which was reverted in
9572388 because it caused formatting not to be enforced for several patches.
The problem was that we'd incorrectly pass the extensions with additional
quoting to git-clang-format. The incorrect quoting has been removed in this
version of the patch.
b3c450d4dc