nmbug: only push master branch on nmbug push

nmbug: only push master branch on nmbug push

nmbug pull only merges upstream master, but nmbug push tries to push
all local branches. The asymmetry results in conflicts whenever there
have been changes in the config branch in the origin:

$ nmbug push
To nmbug@nmbug.tethera.net:nmbug-tags
 ! [rejected]        config -> config (non-fast-forward)
error: failed to push some refs to 'nmbug@nmbug.tethera.net:nmbug-tags'
hint: Updates were rejected because a pushed branch tip is behind its remote
hint: counterpart. If you did not intend to push that branch, you may want to
hint: specify branches to push or set the 'push.default' configuration
hint: variable to 'current' or 'upstream' to push only the current branch.
'git push origin' exited with nonzero value

To fix this, only push the master branch on nmbug push. Any config
changes need to be done manually via git anyway.