This avoids warnings when find_catalog_source_files returns files
found in .git/ like:
WARNING: Start of line didn't match any expected keyword./refs/heads/freezed-library/colorsys.mo
WARNING: Problem on line 1: 0000000000000000000000000000000000000000 032a00a7391ec8a155bbc54d66ba72fb1b7bdbf1 Julien Palard <julien@palard.fr> 1521582790 +0100 branch: Created from freezed/library/colorsys.po
`.po` files can typically reside in .git/refs/heads/ if some branches
name end with .po, in which case they contain a SHA1, not
translations, leading to warnings.