[ci] Skip clang-format for disabled directories

Search for ".clang-format" files with "DisableFormat" lines and remove
any files whose path is below their location (i.e. their path with the
filename removed).

This reduces the runtime of the clang-format CI step by ~5 minutes,
because it takes that long for clang-format to work through the json.h
library header.

.github/workflows/check-and-build.yml

@@ -32,6 +32,8 @@ jobs:
           set -x -u -o pipefail
           clang-format --version
           git ls-files '*.c' '*.cpp' '*.h' \
+              | grep -vf <(grep -rl "^DisableFormat: *true" **/.clang-format \
+                           | sed 's:/.clang-format::')
               | xargs clang-format "${clang_format_args[@]}"
           git diff --exit-code  # i.e. fail CI on non-empty diff