nmbug-status: Use 'show-ref --heads' for loading configs

When loading configs from Git, the bare branch name (without a
refs/heads/ prefix or similar) matches all branches of that name
(including remote-tracking branches):

  .nmbug $ git show-ref config
  48f3bbf1d1492e5f3d2f01de6ea79a30d3840f20 refs/heads/config
  48f3bbf1d1492e5f3d2f01de6ea79a30d3840f20 refs/remotes/origin/config
  4b6dbd9ffd152e7476f5101eff26747f34497cee refs/remotes/wking/config

Instead of relying on the ordering of the matching references, use
--heads to ensure we only match local branches.

NEWS

@@ -23,6 +23,21 @@ Contrib
 previously provided by `notmuch-deliver` should now be provided by
 `notmuch insert`, provided by the main notmuch binary.
 
+nmbug-status
+------------
+
+`nmbug-status` now only matches local branches when reading
+`status-config.json` from the `config` branch of the `NMBGIT`
+repository.  To help new users running `nmbug-status`, `nmbug clone`
+now creates a local `config` branch tracking `origin/config`.  Folks
+who use `nmbug-status` with an in-Git config (i.e. you don't use the
+`--config` option) who already have `NMBGIT` set up are encouraged to
+run:
+
+    git checkout config origin/config
+
+in their `NMBGIT` repository (usually `~/.nmbug`).
+
 Notmuch 0.19 (2014-11-14)
 =========================
 

devel/nmbug/nmbug

@@ -308,6 +308,7 @@ def clone(repository):
             wait=True)
     _git(args=['config', '--unset', 'core.worktree'], wait=True)
     _git(args=['config', 'core.bare', 'true'], wait=True)
+    _git(args=['branch', 'config', 'origin/config'], wait=True)
 
 
 def _is_committed(status):

devel/nmbug/nmbug-status

@@ -92,7 +92,7 @@ def read_config(path=None, encoding=None):
 
         # read only the first line from the pipe
         sha1_bytes = subprocess.Popen(
-            ['git', '--git-dir', nmbhome, 'show-ref', '-s', branch],
+            ['git', '--git-dir', nmbhome, 'show-ref', '-s', '--heads', branch],
             stdout=subprocess.PIPE).stdout.readline()
         sha1 = sha1_bytes.decode(encoding).rstrip()
         if not sha1: