Start with a file array with a resonable preallocated size

Start with a file array with a resonable preallocated size

g_array_new() doesn't reserve any size by default, so during the initial population
of the file array, we'll do more reallocs than strictly needed.  We'll start with
a reasonable preallocated size, in this case the number of files which we can
get in a single chunk out of GIO.

Signed-off-by: Federico Mena Quintero <federico@novell.com>