git-gui: mark *.po files at any directory level as UTF-8

git-gui: mark *.po files at any directory level as UTF-8

When a commit is viewed in Gitk that changes a file in po/glossary, the
patch text shows mojibake instead of correctly decoded UTF-8 text.
Gitk retrieves the encoding attribute to decide how to treat the bytes
that make up the patch text. There is an attribute definition that all
files are US-ASCII, and a later attribute definition overrides this.
But the override, which specifies UTF-8, applies only to *.po files in
directory po/ and does not apply to subdirectories.

Widen the pattern to apply to all directory levels.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>