### Please describe the problem.

annex get does not work from read-only file systems...

### What steps will reproduce the problem?

    $ git annex get --from=...
    error: could not lock config file /.../Annex/.git/config: Read-only file system
    get ... (from ...) error: could not lock config file .../Annex/.git/config: Read-only file system
      git [Param "config",Param "annex.version",Param "5"] failed
    failed

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

annex.version = 3 in the remote

    $ git annex version
    git-annex version: 5.20140927
    build flags: Assistant Webapp Webapp-secure Pairing Testsuite S3 WebDAV Inotify DBus DesktopNotify XMPP DNS Feeds Quvi TDFA CryptoHash
    key/value backends: SHA256E SHA1E SHA512E SHA224E SHA384E SKEIN256E SKEIN512E SHA256 SHA1 SHA512 SHA224 SHA384 SKEIN256 SKEIN512 WORM URL
    remote types: git gcrypt S3 bup directory rsync web webdav tahoe glacier ddar hook external
    local repository version: 5
    supported repository version: 5
    upgrade supported from repository versions: 0 1 2 4

[[!tag confirmed]]
[[!meta title="read-only filesystem on remote prevents auto-upgrade from v3 to v5, and prevents using a remote"]]

> [[done]]; Fixed for V3 mode and rejected trying to support all old
> versions without a repo-changing upgrade process. --[[Joey]]
