Several improvements
- Only add files that we actually want to format.
- Sort according to a fixed locale.
Some drive-by fixes
- Rename a text file, this avoids a filter exception.
- Adds a some missing source files extensions.
- Removes unused extensions hh, hxx, cc, and cxx from clang-format.
AFAIK we don't have most of these file endings. IMO we shouldn't list them here (and remove them from check-format).