miner-fs: use GFile to check for homedir

Instead of a simple path comparison. This will work with relative paths,
double slashes and similar scenarios, since the path will be
fully-canonicalized by GIO before comparison.
The price to pay is recursively indexing the home directory, so we'd
better be extra careful...

https://bugzilla.gnome.org/show_bug.cgi?id=741317