stats.sh: process files in memory instead of on disk

stats.sh: process files in memory instead of on disk

Writing the file to the filesystem for all tags is unnecessary if the
content can just be parsed in memory. This improves the performance:

Here the output of `time ./stats.sh`:

old
./stats.sh  5.96s user 26.53s system 94% cpu 34.533 total

new
./stats.sh  5.85s user 22.86s system 102% cpu 28.040 total

So a reduction of 6.5s (23%) is achieved even on a computer with a
decent SSD.