### Please describe the problem.

I'm playing with git-annex on Android and I see the following error messages for file names starting with `-`:

    Usage: git-annex smudge (FILE [--clean] | --update)
    error: external filter 'git-annex smudge %f' failed 1
    error: external filter 'git-annex smudge %f' failed
    Invalid option `-123min/Home/9 - Home.mp3'

### What steps will reproduce the problem?

1. Add a file starting with `-` to the annex (on Debian/Linux in my case).
2. On another system (Android in my case) add `git remote` and run `git annex sync`.
3. Observe the error messages.

### What version of git-annex are you using? On what operating system?

    git-annex version: 7.20181122-gbb94cc9f8
    build flags: Assistant Webapp Pairing S3(multipartupload)(storageclasses) WebDAV Inotify DBus DesktopNotify TorrentParser MagicMime Feeds Testsuite
    dependency versions: aws-0.19 bloomfilter-2.0.1.0 cryptonite-0.25 DAV-1.3.2 feed-1.0.0.0 ghc-8.2.2 http-client-0.5.12 persistent-sqlite-2.8.1.2 torrent-10000.1.1 uuid-1.3.13 yesod-1.6.0
    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 BLAKE2S256E BLAKE2S256 BLAKE2S160E 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: linux aarch64
    supported repository versions: 5 7
    upgrade supported from repository versions: 0 1 2 3 4 5 6
    local repository version: 7

### 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)

Sure :)
