Merge branch 'rr/completion-branch-config'

Two-level configuration variable names in "branch.*" and "remote.*"
hierarchies whose variables are predominantly three-level where not
completed by hitting a <TAB> in bash and zsh completions.

* rr/completion-branch-config:
completion: fix remote.pushdefault
completion: fix branch.autosetup(merge|rebase)
completion: introduce __gitcomp_nl_append ()
zsh completion: find matching custom bash completion