### Please describe the problem.
not work: Git annex addurl existing file without redownloading, use URL-backend (not any hashkey-backend)

### What steps will reproduce the problem?
File copied to git (git-annex) repo's dir  (did not 'git add' 'git annex add')
name: 'f.mp4'
Now run 'git annex addurl --file=f.mp4 --raw --relaxed https://www.youtube.com/watch?v=U33dsEcKge ' (via Python, see below)

RESULT:
failed "f.mp4 already exists; not overwriting"

(same if run in Python)
File "/opt/anaconda3/lib/python3.7/subprocess.py", line 512, in run
    output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '['git', 'annex', 'addurl', '--file=f.mp4', '--raw', '--relaxed', 'https://www.youtube.com/watch?v=U33dsEcKgeQ']' returned non-zero exit status 1.

Above command would work after doing 'git annex add f.mp4' first
  but it results in a backend not 'URL backend for youtube'
  I'd like to use 'URL backend for youtube' cause I worry about youtube-video binay-change, in which case all future download will fail backend verification.



### What version of git-annex are you using? On what operating system?
macOS 10.14.6, installed by 'brew install git-annex'
git annex version
git-annex version: 8.20201129
build flags: Assistant Webapp Pairing FsEvents TorrentParser MagicMime Feeds Testsuite S3 WebDAV
dependency versions: aws-0.22 bloomfilter-2.0.1.0 cryptonite-0.27 DAV-1.3.4 feed-1.3.0.1 ghc-8.10.3 http-client-0.7.3 persistent-sqlite-2.11.0.0 torrent-10000.1.1 uuid-1.3.13 yesod-1.6.1.0
key/value backends: SHA256E SHA256 SHA512E SHA512 SHA224E SHA224 SHA384E SHA384 SHA3_256E SHA3_256 SHA3_512E SHA3_512 SHA3_224E SHA3_224 SHA3_384E SHA3_384 SKEIN256E SKEIN256 SKEIN512E SKEIN512 BLAKE2B256E BLAKE2B256 BLAKE2B512E BLAKE2B512 BLAKE2B160E BLAKE2B160 BLAKE2B224E BLAKE2B224 BLAKE2B384E BLAKE2B384 BLAKE2BP512E BLAKE2BP512 BLAKE2S256E BLAKE2S256 BLAKE2S160E BLAKE2S160 BLAKE2S224E BLAKE2S224 BLAKE2SP256E BLAKE2SP256 BLAKE2SP224E BLAKE2SP224 SHA1E SHA1 MD5E MD5 WORM URL X*
remote types: git gcrypt p2p S3 bup directory rsync web bittorrent webdav adb tahoe glacier ddar git-lfs httpalso borg hook external
operating system: darwin x86_64
supported repository versions: 8
upgrade supported from repository versions: 0 1 2 3 4 5 6 7


### Please provide any additional information below.
command line taken from https://git-annex.branchable.com/tips/using_the_web_as_a_special_remote/

[[!format sh """
# If you can, paste a complete transcript of the problem occurring here.
# If the problem is with the git-annex assistant, paste in .git/annex/daemon.log


# End of transcript or log.
"""]]

### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)
oh, yes, git-annex is great for managing binary files, thanks for the work!

> [[done]], see my comment --[[Joey]]
