nmbug: Accept failures to unset core.worktree in clone

nmbug: Accept failures to unset core.worktree in clone

Since 6311cfaf (init: do not set unnecessary core.worktree,
2016-09-25, 2.11.0 [1]), Git has no longer set core.worktree when
--separate-git-dir is used.  This broke clone with:

  $ nmbug clone http://nmbug.notmuchmail.org/git/nmbug-tags.git
  Cloning into '/tmp/nmbug-clone.33gg442e'...
  Checking connectivity: 16674, done.
  ['git', '--git-dir', '/home/wking/.nmbug', 'config', '--unset', 'core.worktree'] exited with 5
  $ echo $?
  1

The initial discussion that lead to the Git change is in [2], and
there is some more discussion around this specific change in [3].
There is some useful background on working trees in this 2009 message
[4].  There is also a git-worktree(1) since df0b6cfb (worktree: new
place for "git prune --worktrees", 2015-06-29, 2.5.0 [5]) which grew
the ability to add new worktrees in 799767cc (Merge branch
'es/worktree-add', 2015-07-13, 2.5.0 [6]).  Folks relying on
core.worktree in the --separate-git-dir case fall into the "former
case" in [4], and as Junio pointed out in that message, Git
operations like 'add' don't really work there.

In nmbug we don't want core.worktree, because our effective working
tree is the notmuch database.  By accepting failed core.worktree
unsets, clone will work with Gits older and younger than 2.11.0.

[1]: https://github.com/git/git/commit/6311cfaf93716bcc43dd1151cb1763e3f80d8099
[2]: https://public-inbox.org/git/CALqjkKZO_y0DNcRJjooyZ7Eso7yBMGhvZ6fE92oO4Su7JeCeng@mail.gmail.com/
[3]: https://public-inbox.org/git/87h94d8cwi.fsf@kyleam.com/
[4]: https://public-inbox.org/git/7viqbsw2vn.fsf@alter.siamese.dyndns.org/
[5]: https://github.com/git/git/commit/df0b6cfbda88144714541664fb501146d6465a82
[6]: https://github.com/git/git/commit/799767cc98b2f8e6f82d0de4bef9b5e8fcc16e97

Reported-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>