nmbug: Add 'clone' and replace FETCH_HEAD with @{upstream}

With two branches getting fetched (master and config), the branch
referenced by FETCH_HEAD is ambiguous.  For example, I have:

  $ cat FETCH_HEAD
  41d7bfa7184cc93c9dac139d1674e9530799e3b0 \
    not-for-merge   branch 'config' of http://nmbug.tethera.net/git/nmbug-tags
  acd379ccb973c45713eee9db177efc530f921954 \
    not-for-merge   branch 'master' of http://nmbug.tethera.net/git/nmbug-tags

(where I wrapped the line by hand).  This means that FETCH_HEAD
references the config branch:

  $ git rev-parse FETCH_HEAD
  41d7bfa7184cc93c9dac139d1674e9530799e3b0

which breaks all of the FETCH_HEAD logic in nmbug (where FETCH_HEAD is
assumed to point to the master branch).

Instead of relying on FETCH_HEAD, use @{upstream} as the
remote-tracking branch that should be merged/diffed/integrated into
HEAD.  @{upstream} was added in Git v1.7.0 (2010-02-12) [1], so
relying on it should be fairly safe.  One tricky bit is that bare
repositories don't set upstream tracking branches by default:

  $ git clone --bare http://nmbug.tethera.net/git/nmbug-tags.git nmbug-bare
  $ cd nmbug-bare
  $ git remote show origin
  * remote origin
    Fetch URL: http://nmbug.tethera.net/git/nmbug-tags.git
    Push  URL: http://nmbug.tethera.net/git/nmbug-tags.git
    HEAD branch: master
    Local refs configured for 'git push':
      config pushes to config (up to date)
      master pushes to master (up to date)

While in a non-bare clone:

  $ git clone http://nmbug.tethera.net/git/nmbug-tags.git
  $ cd nmbug-tags
  $ git remote show origin
  * remote origin
    Fetch URL: http://nmbug.tethera.net/git/nmbug-tags.git
    Push  URL: http://nmbug.tethera.net/git/nmbug-tags.git
    HEAD branch: master
    Remote branches:
      config tracked
      master tracked
    Local branch configured for 'git pull':
      master merges with remote master
    Local ref configured for 'git push':
      master pushes to master (up to date)

From the clone docs [2]:

  --bare::
        Make a 'bare' Git repository…
        Also the branch heads at the remote are copied directly
        to corresponding local branch heads, without mapping
        them to `refs/remotes/origin/`.  When this option is
        used, neither remote-tracking branches nor the related
        configuration variables are created.

To use @{upstream}, we need to the local vs. remote-tracking
distinction, so this commit adds 'nmbug clone', replacing the
previously suggested --bare clone with a non-bare --no-checkout
--separate-git-dir clone into a temporary work directory.  After
which:

  $ git rev-parse @{upstream}
  acd379ccb973c45713eee9db177efc530f921954

gives us the master-branch commit.  Existing nmbug users will have to
run the configuration tweaks and re-fetch by hand.  If you don't have
any local commits, you could also blow away your NMBGIT repository and
re-clone from scratch:

  $ nmbug clone http://nmbug.tethera.net/git/nmbug-tags.git

Besides removing the ambiguity of FETCH_HEAD, this commit allows users
to configure which upstream branch they want nmbug to track via 'git
config', in case they want to change their upstream repository.

[1]: http://git.kernel.org/cgit/git/git.git/tree/Documentation/RelNotes/1.7.0.txt
[2]: http://git.kernel.org/cgit/git/git.git/tree/Documentation/git-clone.txt

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