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

devel/nmbug/nmbug

@@ -26,6 +26,7 @@ my $ESCAPED_RX =	qr{$ESCAPE_CHAR([A-Fa-f0-9]{2})};
 my %command = (
 	     archive	=> \&do_archive,
 	     checkout	=> \&do_checkout,
+	     clone	=> \&do_clone,
 	     commit	=> \&do_commit,
 	     fetch	=> \&do_fetch,
 	     help	=> \&do_help,
@@ -125,6 +126,15 @@ sub do_archive {
   system ('git', "--git-dir=$NMBGIT", 'archive', 'HEAD');
 }
 
+sub do_clone {
+  my $repository = shift;
+
+  my $tempwork = tempdir ('/tmp/nmbug-clone.XXXXXX', CLEANUP => 1);
+  system ('git', 'clone', '--no-checkout', '--separate-git-dir', $NMBGIT,
+          $repository, $tempwork) == 0
+    or die "'git clone' exited with nonzero value\n";
+  git ('config', '--unset', 'core.worktree');
+}
 
 sub is_committed {
   my $status = shift;
@@ -332,21 +342,24 @@ To discard your changes,  run 'nmbug checkout'
 
 sub do_pull {
   my $remote = shift || 'origin';
+  my $branch = shift || 'master';
 
   git ( 'fetch', $remote);
 
-  do_merge ();
+  do_merge ("$remote/$branch");
 }
 
 
 sub do_merge {
+  my $commit = shift || '@{upstream}';
+
   insist_committed ();
 
   my $tempwork = tempdir ('/tmp/nmbug-merge.XXXXXX', CLEANUP => 1);
 
   git ( { GIT_WORK_TREE => $tempwork }, 'checkout', '-f', 'HEAD');
 
-  git ( { GIT_WORK_TREE => $tempwork }, 'merge', 'FETCH_HEAD');
+  git ( { GIT_WORK_TREE => $tempwork }, 'merge', $commit);
 
   do_checkout ();
 }
@@ -407,11 +420,10 @@ sub do_status {
 
 
 sub is_unmerged {
+  my $commit = shift || '@{upstream}';
 
-  return 0 if (! -f $NMBGIT.'/FETCH_HEAD');
-
-  my $fetch_head = git ('rev-parse', 'FETCH_HEAD');
-  my $base = git ( 'merge-base', 'HEAD', 'FETCH_HEAD');
+  my $fetch_head = git ('rev-parse', $commit);
+  my $base = git ( 'merge-base', 'HEAD', $commit);
 
   return ($base ne $fetch_head);
 
@@ -473,7 +485,7 @@ sub diff_index {
 sub diff_refs {
   my $filter = shift;
   my $ref1 = shift || 'HEAD';
-  my $ref2 = shift || 'FETCH_HEAD';
+  my $ref2 = shift || '@{upstream}';
 
   my $fh= git_pipe ( 'diff', "--diff-filter=$filter", '--name-only',
 		 $ref1, $ref2);
@@ -561,10 +573,11 @@ git. Any extra arguments are used (one per line) as a commit message.
 
 push local nmbug git state to remote repo
 
-=item  B<pull> [remote]
+=item  B<pull> [remote] [branch]
 
 pull (merge) remote repo changes to notmuch. B<pull> is equivalent to
-B<fetch> followed by B<merge>.
+B<fetch> followed by B<merge>.  The default remote is C<origin>, and
+the default branch is C<master>.
 
 =back
 
@@ -572,6 +585,12 @@ B<fetch> followed by B<merge>.
 
 =over 8
 
+=item B<clone> repository
+
+Create a local nmbug repository from a remote source.  This wraps
+C<git clone>, adding some options to avoid creating a working tree
+while preserving remote-tracking branches and upstreams.
+
 =item B<checkout>
 
 Update the notmuch database from git. This is mainly useful to discard
@@ -589,12 +608,12 @@ print help [for subcommand]
 =item B<log> [parameters]
 
 A simple wrapper for git log. After running C<nmbug fetch>, you can
-inspect the changes with C<nmbug log HEAD..FETCH_HEAD>
+inspect the changes with C<nmbug log HEAD..@{upstream}>
 
-=item B<merge>
+=item B<merge> [commit]
 
-Merge changes from FETCH_HEAD into HEAD, and load the result into
-notmuch.
+Merge changes from C<commit> into HEAD, and load the result into
+notmuch.  The default commit is C<@{upstream}>.
 
 =item  B<status>