Add hub's completion file

This uses the version at commit 35c76bf (2015-12-08)
https://github.com/github/hub/commit/35c76bf91dc3d699173599464b9b41cb877ee440

plugins/github/_hub

+#compdef hub
+
+# Zsh will source this file when attempting to autoload the "_hub" function,
+# typically on the first attempt to complete the hub command.  We define two new
+# setup helper routines (one for the zsh-distributed version, one for the
+# git-distributed, bash-based version).  Then we redefine the "_hub" function to
+# call "_git" after some other interception.
+#
+# This is pretty fragile, if you think about it.  Any number of implementation
+# changes in the "_git" scripts could cause problems down the road.  It would be
+# better if the stock git completions were just a bit more permissive about how
+# it allowed third-party commands to be added.
+
+(( $+functions[__hub_setup_zsh_fns] )) ||
+__hub_setup_zsh_fns () {
+  (( $+functions[_git-alias] )) ||
+  _git-alias () {
+    _arguments \
+      '-s[output shell script suitable for eval]' \
+      '1::shell:(zsh bash csh)'
+  }
+
+  (( $+functions[_git-browse] )) ||
+  _git-browse () {
+    _arguments \
+      '-u[output the URL]' \
+      '2::subpage:(wiki commits issues)'
+  }
+
+  (( $+functions[_git-compare] )) ||
+  _git-compare () {
+    _arguments \
+      '-u[output the URL]' \
+      ':[start...]end range:'
+  }
+
+  (( $+functions[_git-create] )) ||
+  _git-create () {
+    _arguments \
+      '::name (REPOSITORY or ORGANIZATION/REPOSITORY):' \
+      '-p[make repository private]' \
+      '-d[description]:description' \
+      '-h[home page]:repository home page URL:_urls'
+  }
+
+  (( $+functions[_git-fork] )) ||
+  _git-fork () {
+    _arguments \
+      '--no-remote[do not add a remote for the new fork]'
+  }
+
+  (( $+functions[_git-pull-request] )) ||
+  _git-pull-request () {
+    _arguments \
+      '-f[force (skip check for local commits)]' \
+      '-b[base]:base ("branch", "owner\:branch", "owner/repo\:branch"):' \
+      '-h[head]:head ("branch", "owner\:branch", "owner/repo\:branch"):' \
+      - set1 \
+        '-m[message]' \
+        '-F[file]' \
+        '-a[user]' \
+        '-M[milestone]' \
+        '-l[labels]' \
+      - set2 \
+        '-i[issue]:issue number:' \
+      - set3 \
+        '::issue-url:_urls'
+  }
+
+  # stash the "real" command for later
+  functions[_hub_orig_git_commands]=$functions[_git_commands]
+
+  # Replace it with our own wrapper.
+  declare -f _git_commands >& /dev/null && unfunction _git_commands
+  _git_commands () {
+    local ret=1
+    # call the original routine
+    _call_function ret _hub_orig_git_commands
+
+    # Effectively "append" our hub commands to the behavior of the original
+    # _git_commands function.  Using this wrapper function approach ensures
+    # that we only offer the user the hub subcommands when the user is
+    # actually trying to complete subcommands.
+    hub_commands=(
+      alias:'show shell instructions for wrapping git'
+      pull-request:'open a pull request on GitHub'
+      fork:'fork origin repo on GitHub'
+      create:'create new repo on GitHub for the current project'
+      browse:'browse the project on GitHub'
+      compare:'open GitHub compare view'
+      ci-status:'lookup commit in GitHub Status API'
+    )
+    _describe -t hub-commands 'hub command' hub_commands && ret=0
+
+    return ret
+  }
+}
+
+(( $+functions[__hub_setup_bash_fns] )) ||
+__hub_setup_bash_fns () {
+  # TODO more bash-style fns needed here to complete subcommand args.  They take
+  # the form "_git_CMD" where "CMD" is something like "pull-request".
+
+  # Duplicate and rename the 'list_all_commands' function
+  eval "$(declare -f __git_list_all_commands | \
+        sed 's/__git_list_all_commands/__git_list_all_commands_without_hub/')"
+
+  # Wrap the 'list_all_commands' function with extra hub commands
+  __git_list_all_commands() {
+    cat <<-EOF
+alias
+pull-request
+fork
+create
+browse
+compare
+ci-status
+EOF
+    __git_list_all_commands_without_hub
+  }
+
+  # Ensure cached commands are cleared
+  __git_all_commands=""
+}
+
+# redefine _hub to a much smaller function in the steady state
+_hub () {
+  # only attempt to intercept the normal "_git" helper functions once
+  (( $+__hub_func_replacement_done )) ||
+    () {
+      # At this stage in the shell's execution the "_git" function has not yet
+      # been autoloaded, so the "_git_commands" or "__git_list_all_commands"
+      # functions will not be defined.  Call it now (with a bogus no-op service
+      # to prevent premature completion) so that we can wrap them.
+      if declare -f _git >& /dev/null ; then
+        _hub_noop () { __hub_zsh_provided=1 }       # zsh-provided will call this one
+        __hub_noop_main () { __hub_git_provided=1 } # git-provided will call this one
+        local service=hub_noop
+        _git
+        unfunction _hub_noop
+        unfunction __hub_noop_main
+        service=git
+      fi
+
+      if (( $__hub_zsh_provided )) ; then
+        __hub_setup_zsh_fns
+      elif (( $__hub_git_provided )) ; then
+        __hub_setup_bash_fns
+      fi
+
+      __hub_func_replacement_done=1
+    }
+
+  # Now perform the actual completion, allowing the "_git" function to call our
+  # replacement "_git_commands" function as needed.  Both versions expect
+  # service=git or they will call nonexistent routines or end up in an infinite
+  # loop.
+  service=git
+  declare -f _git >& /dev/null && _git
+}
+
+# make sure we actually attempt to complete on the first "tab" from the user
+_hub