Update doesn't respect default value of locale_dirs from `-c` passed `conf.py`

[liborjelinek]

If conf.py doesn't explicitly set locale_dirs, it's ["locales"].

When I pass such conf.py to sphinx-intl update command, it incorrectly puts .po files to <cwd>/locales/<lang>/LC_MESSAGES/ (under currently working directory).

But the paths of locale_dirs are relative to conf.py, i.e. it should puts .po files to <conf.py dir>/locales/<lang>/LC_MESSAGES/ instead.

For this buggy behavior, you need to explicitly set locale_dirs = ["locales/"] in your conf.py, i.e., to its default value.