A few "linked checkout" niggles

[Junio C Hamano <gitster@pobox.com>]

I've tried to set up a non-bare clone of git.git at ~/w/src and
attached one of its branches to a separate work tree at ~/w/rerere

 ~/w/src$ git worktree add ../rerere jc/rerere

Then I tried the "multiple checkout not allowed".

 ~/w/src$ git checkout jc/rerere
 fatal: 'jc/rerere' is already checked out at '/home/gitster/w/rerere/.git'

You see the first "niggle" above.  Do we need "/.git" at the end?
That is hopefully merely cosmetic and I am guessing that we can
unconditionally drop "/.git" from the end, but maybe there are
subtle reasons why it may not be a good idea (e.g. introduces some
ambiguities?)

There is another, a more serious one.

 ~/w/src$ git fetch ../git.git jc/rerere:jc/rerere

This succeeds, without checking if there is a work tree that has it
checked out.  I think the safety in "git fetch" that can be disabled
by "--update-head-ok" option should be updated to reuse the same
protection used by the updated "git checkout".

Something to work on for 2.5.x maintenance track, aiming for 2.5.1.

Thanks.