--get-filename returns incorrect filename if output file gets merged

[nixxquality]

• I've verified and I assure that I'm running youtube-dl 2016.12.15
• At least skimmed through README and most notably FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
• Feature request (request for a new functionality)
• Question
• Other

$ youtube-dl --get-filename https://www.youtube.com/watch?v=HEv_9fQBWVY
CrossFit for Chickens-HEv_9fQBWVY.mp4
$ youtube-dl https://www.youtube.com/watch?v=HEv_9fQBWVY
[youtube] HEv_9fQBWVY: Downloading webpage
[youtube] HEv_9fQBWVY: Downloading video info webpage
[youtube] HEv_9fQBWVY: Extracting video information
[youtube] HEv_9fQBWVY: Downloading MPD manifest
WARNING: Requested formats are incompatible for merge and will be merged into mkv.
[download] Destination: CrossFit for Chickens-HEv_9fQBWVY.f136.mp4
[download] 100% of 16.32MiB in 00:00
[download] Destination: CrossFit for Chickens-HEv_9fQBWVY.f251.webm
[download] 100% of 1.36MiB in 00:00
[ffmpeg] Merging formats into "CrossFit for Chickens-HEv_9fQBWVY.mkv"
Deleting original file CrossFit for Chickens-HEv_9fQBWVY.f136.mp4 (pass -k to keep)
Deleting original file CrossFit for Chickens-HEv_9fQBWVY.f251.webm (pass -k to keep)

The --get-filename function should check if mkv merge is needed, and return a proper filename.

[dstftw]

Duplicate of #5710.