improve log processing to note combined files

[tardisx]

For example:

[youtube] FTQbiNvZqaY: Downloading webpage
[info] Writing video description metadata as JSON to: Toto - Africa (Official HD Video)-FTQbiNvZqaY.info.json
[download] Destination: Toto - Africa (Official HD Video)-FTQbiNvZqaY.f137.mp4
[download]   0.0% of 48.45MiB at 659.38KiB/s ETA 01:16
[download]   0.0% of 48.45MiB at  1.76MiB/s ETA 00:27
[download]   0.0% of 48.45MiB at  3.92MiB/s ETA 00:12
...
[download]  96.9% of 48.45MiB at  3.94MiB/s ETA 00:00
[download] 100.0% of 48.45MiB at  4.18MiB/s ETA 00:00
[download] 100% of 48.45MiB in 00:08
[download] Destination: Toto - Africa (Official HD Video)-FTQbiNvZqaY.f140.m4a
[download]   0.0% of 4.19MiB at 298.12KiB/s ETA 00:14
[download]   0.1% of 4.19MiB at 856.97KiB/s ETA 00:05
...
[download]  95.4% of 4.19MiB at  5.96MiB/s ETA 00:00
[download] 100.0% of 4.19MiB at  5.99MiB/s ETA 00:00
[download] 100% of 4.19MiB in 00:00
[ffmpeg] Merging formats into "Toto - Africa (Official HD Video)-FTQbiNvZqaY.mp4"
Deleting original file Toto - Africa (Official HD Video)-FTQbiNvZqaY.f137.mp4 (pass -k to keep)
Deleting original file Toto - Africa (Official HD Video)-FTQbiNvZqaY.f140.m4a (pass -k to keep)

We should remove the two deleted files from the list as they appear in the log, and add the "Merging" filename.