adjust setting of git-user

Also deactivate ensurance of `authorized_keys`
file existence (since it won't work).
fix pathname related issue.
use current-symlink to refer to git-shell.
only fail on remove, when file not exists.
delete key from authorized_keys-file correctly.
extract authorized_keys open into own method

Also lock the file while working on it.
fix strange syntax error with method evaluation.
adjust generation of auhtorized_key line

The options can't have whitespace between the
commata.
fix file truncation issue.
fix escaping of git-shell command.
fix preserving other keylines on key-delete

When deleting a Key we should only
delete its line in the authorized_keys file.
An issue with File#lines resulted in the
whole file being truncated.
fix matching of key lines

also remove unecessary closing of authorized_keys file,
since we are using a block and the file will be
closed automatically.
fix rewriting of authorized_keys when removing.
fix load of rails_config for git-shell.
adjust gem-loading for rails_config in git-shell.
fix GEM_HOME path for git-shell.
add relation between ssh-controller and repository.
use special finder for repository in ssh_access.
use git_home instead of git_root

git_root is the actual directory where the bare
git repositories are stored. git_home is the directory
with the bare-git-repo symlinks, which are named
after the name of the repository instead of its id.
use git-ssh symlink-dir for repositories.