Don't run when no file is being renamed after a `modify` call

[FichteFoll]

Following is the console output of me setting some flexattrs on an item that do not result in any file being moved. However, extrafiles still shows a message that it didn't do any moving.

It should probably be checked whether the source and the destination of a file's move action are the same.

~ Σ beet modify ^title:inst ^title:TV ^title:vocal ^title:karaoke title:'Genesis' anime=Opening source="Dimension W" STEREO
/usr/lib/python3.8/site-packages/beets/mediafile.py:21: UserWarning: beets.mediafile is deprecated; use mediafile instead
  warnings.warn("beets.mediafile is deprecated; use mediafile instead")
Modifying 1 items.
STEREO DIVE FOUNDATION - Genesis - Genesis
  source: Dimension W
  anime: Opening
Really modify, move and write tags? (Yes/no/select) y

extrafiles: Skipping already present destination file: /data/audio/music/sorted/STEREO DIVE FOUNDATION/2016 - Genesis/cover.jpg

Related: #6