### Please describe the problem.

Unable to addurl to a `file:///` on Windows

1. doesn't understand `file:///C:/`
2. with `file://C:/` blows with permission denied:

[[!format sh """
C:\...pData\Local\Temp\1\datalad_temp_testrepo_tmphjl88>git annex addurl --file buga file:///C:/123
addurl file:///C:/123
download failed: /C:/123: openBinaryFile: invalid argument (Invalid argument)
failed
git-annex: addurl: 1 failed

C:\...pData\Local\Temp\1\datalad_temp_testrepo_tmphjl88>git annex addurl --file buga file://C:/123
addurl file://C:/123
(to buga)
git-annex: .git\annex\tmp\URL-s6--file&c%%C&c%123: renameFile:renamePath:MoveFileEx "\\\\?\\C:\\Users\\appveyor\\
AppData\\Local\\Temp\\1\\datalad_temp_testrepo_tmphjl88\\.git\\annex\\tmp\\URL-s6--file&c%%C&c%123" Just "\\\\?\\
C:\\Users\\appveyor\\AppData\\Local\\Temp\\1\\datalad_temp_testrepo_tmphjl88\\buga": permission denied (The proce
ss cannot access the file because it is being used by another process.)
failed
git-annex: addurl: 1 failed

"""]]

here is some relevant details (and showing curl handling both file:// and file:///):
[[!format sh """
C:\...pData\Local\Temp\1\datalad_temp_testrepo_tmphjl88>git status
On branch adjusted/master(unlocked)
nothing to commit, working tree clean

C:\...pData\Local\Temp\1\datalad_temp_testrepo_tmphjl88>git annex version
git-annex version: 7.20181205-g51d6f38b1
build flags: Assistant Webapp Pairing S3(multipartupload)(storageclasses) WebDAV TorrentParser Feeds Testsuite
dependency versions: aws-0.17.1 bloomfilter-2.0.1.0 cryptonite-0.23 DAV-1.3.1 feed-0.3.12.0 ghc-8.0.2 http-client
-0.5.7.1 persistent-sqlite-2.6.2 torrent-10000.1.1 uuid-1.3.13 yesod-1.4.5
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 BLAKE2B51
2E BLAKE2B512 BLAKE2B160E BLAKE2B160 BLAKE2B224E BLAKE2B224 BLAKE2B384E BLAKE2B384 BLAKE2S256E BLAKE2S256 BLAKE2S
160E BLAKE2S160 BLAKE2S224E BLAKE2S224 BLAKE2SP256E BLAKE2SP256 BLAKE2SP224E BLAKE2SP224 SHA1E SHA1 MD5E MD5 WORM
 URL
remote types: git gcrypt p2p S3 bup directory rsync web bittorrent webdav adb tahoe glacier ddar hook external
operating system: mingw32 i386
supported repository versions: 5 7
upgrade supported from repository versions: 2 3 4 5 6
local repository version: 7

C:\...pData\Local\Temp\1\datalad_temp_testrepo_tmphjl88>git status
On branch adjusted/master(unlocked)
nothing to commit, working tree clean

C:\...pData\Local\Temp\1\datalad_temp_testrepo_tmphjl88>curl file://C:/123
124

C:\...pData\Local\Temp\1\datalad_temp_testrepo_tmphjl88>curl file:///C:/123
124
"""]]

More information about this appveyor server could be obtained from [datalad wtf](http://paste.debian.net/1055359/) output

Awhile back we [had related discussion](https://git-annex.branchable.com/bugs/git-annex_drop_fails_to_access_file__58____47____47____47___target_URL_on_Windows/) but at least `addurl` seemed to work then.


[[!meta author=yoh]]
[[!tag projects/repronim]]
