[vcsh] Bijective vcsh test

[Richard Hartmann]

Hi all,

to allow external programs to test if any given fake bare git repo is
managed via vcsh, I introduced vcsh.vcsh as config option in
gitconfig.

init & clone will set this automagically, existing repos can be updated via

    for repo in $(vcsh list); do vcsh setup $repo; done

please note that, due to git's nature, you will need to run this on
all hosts you have existing vcsh repos on.

In the future, if you want to manage existing non-vcsh fake bare repos
with vcsh, it's recommended to run this, as well.


Richard