shell completion: support external commands

Closes Homebrew/homebrew#41519.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Contributions/brew_bash_completion.sh

@@ -580,62 +580,7 @@ _brew ()
     done
 
     if [[ $i -eq $COMP_CWORD ]]; then
-        __brewcomp "
+        __brewcomp "$(brew commands --quiet --include-aliases)"
-            --cache --cellar
-            --env --prefix --repository
-            aspell-dictionaries
-            audit
-            bottle
-            cat
-            cleanup
-            commands
-            config --config
-            create
-            deps
-            diy configure
-            doctor
-            edit
-            fetch
-            gist-logs
-            help
-            home
-            info abv
-            install
-            irb
-            leaves
-            linkapps
-            link ln
-            list ls
-            log
-            man
-            missing
-            options
-            outdated
-            prune
-            pin
-            postinstall
-            pull
-            readall
-            search
-            reinstall
-            sh
-            style
-            switch
-            tap
-            tap-readme
-            test
-            test-bot
-            tests
-            uninstall remove rm
-            unlink
-            unlinkapps
-            unpack
-            unpin
-            untap
-            update
-            upgrade
-            uses
-            "
         return
     fi
 
@@ -677,7 +622,7 @@ _brew ()
     uninstall|remove|rm)        _brew_uninstall ;;
     unpack)                     _brew_unpack ;;
     unpin)                      __brew_complete_formulae ;;
-    untap)                      __brew_complete_tapped ;;
+    untap|tap-info)             __brew_complete_tapped ;;
     update)                     _brew_update ;;
     upgrade)                    _brew_upgrade ;;
     uses)                       _brew_uses ;;

Library/Contributions/brew_zsh_completion.zsh

@@ -25,7 +25,6 @@ _brew_outdated_formulae() {
 local -a _1st_arguments
 _1st_arguments=(
   'audit:check formulae for Homebrew coding style'
-  'bundle:look for a Brewfile and run each line as a brew command'
   'cat:display formula file for a formula'
   'cleanup:uninstall unused and old versions of packages'
   'commands:show a list of commands'
@@ -55,6 +54,7 @@ _1st_arguments=(
   'update:freshen up links'
   'upgrade:upgrade outdated formulae'
   'uses:show formulae which depend on a formula'
+  `brew commands --quiet --include-aliases`
 )
 
 local expl
@@ -97,7 +97,7 @@ case "$words[1]" in
     _arguments \
       '(--macports)--macports[search the macports repository]' \
       '(--fink)--fink[search the fink repository]' ;;
-  untap)
+  untap|tap-info)
     _brew_installed_taps
     _wanted installed_taps expl 'installed taps' compadd -a installed_taps ;;
   upgrade)