* Use user-preferred locale
Previous fix adding `L` format specifier was correct but only fixed half
of the problem, as C++ locale is set up to be "C" by default. GTK client
used to call `setlocale(LC_ALL, "")` to set up user-preferred locale
which only affected C functions and `std::locale` (used by libfmt) was
unaware of those changes.
Apply the fix to all the binaries since they're all doing some sort of
output to the user and calling libtransmission helpers, as well as using
libfmt directly.
* Improve libtransmission's json-test
Set the locale C++ way to avoid any sort of inconsistencies, and also
restore it to the old one once finished testing.
* Improve transmission-show test runner script
Quote outputs to avoid CMake error about `message()` being called with
no arguments. Capture stderr to the same output file. Fallback to `git
diff` if `diff` wasn't found. A few other minor changes.
