info: move __FILE__ path shortening into default log handler

info: move __FILE__ path shortening into default log handler

Instead of always shortening the __FILE__ path, even if the
log message is not actually printed, which might happen if
the log level is activated but the category is not, only
shorten the path if we're actually going to output it and
if it looks like it needs shortening. Log handlers had no
guarantee that they would get a name instead of a path
anyway on any architecture, so it shouldn't be a problem.

https://bugzilla.gnome.org/show_bug.cgi?id=745213