### Please describe the problem.
git-annex prefixes all paths used with SSH remotes with a /, which breaks the default git behavior of using the home directory by default.

### What steps will reproduce the problem?
Start an SSH server. Use it as a remote with a URL like annex@localhost:test.git or annex@localhost:~/test.git.

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

[[!format sh """
[leo60228@nixos:~/test]$ git annex version
git-annex version: 6.20180529
build flags: Assistant Webapp Pairing S3(multipartupload)(storageclasses) WebDAV Inotify DBus DesktopNotify ConcurrentOutput TorrentParser MagicMime Feeds Testsuite
dependency versions: aws-0.18 bloomfilter-2.0.1.0 cryptonite-0.25 DAV-1.3.2 feed-1.0.0.0 ghc-8.2.2 http-client-0.5.12.1 persistent-sqlite-2.6.4 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 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
local repository version: 5
supported repository versions: 3 5 6
upgrade supported from repository versions: 0 1 2 3 4 5
operating system: linux x86_64
"""]]

### Please provide any additional information below.

[[!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

# Don't know how to make a reduced test-case for an issue with a specific protocol.

# 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)
Yes, I use it on my laptop without issues. I'm trying to set up a server on my desktop.
