Speed up brew tap for no arguments

This provides a >3x speedup for `brew tap` with no arguments (i.e., when
listing taps). It also makes the completion significantly faster.

    $ hyperfine --warmup=3 --setup 'git checkout {branch}' --parameter-list branch master,brew-tap-speedup 'brew tap'
    Benchmark 1: brew tap (branch = master)
      Time (mean ± σ):      1.405 s ±  0.080 s    [User: 0.561 s, System: 0.238 s]
      Range (min … max):    1.332 s …  1.549 s    10 runs

    Benchmark 2: brew tap (branch = brew-tap-speedup)
      Time (mean ± σ):     404.1 ms ± 124.8 ms    [User: 107.9 ms, System: 200.7 ms]
      Range (min … max):   308.8 ms … 693.7 ms    10 runs

    Summary
      brew tap (branch = brew-tap-speedup) ran
        3.48 ± 1.09 times faster than brew tap (branch = master)

Library/Homebrew/brew.sh

@@ -172,6 +172,11 @@ case "$@" in
     source "${HOMEBREW_LIBRARY}/Homebrew/list.sh"
     homebrew-list "$@" && exit 0
     ;;
+  # falls back to cmd/tap.rb on a non-zero return
+  tap*)
+    source "${HOMEBREW_LIBRARY}/Homebrew/tap.sh"
+    homebrew-tap "$@" && exit 0
+    ;;
   # falls back to cmd/help.rb on a non-zero return
   help | --help | -h | --usage | "-?" | "")
     homebrew-help "$@" && exit 0

Library/Homebrew/tap.sh

@@ -0,0 +1,41 @@
+# Does the quickest output of brew tap possible for no arguments.
+# HOMEBREW_LIBRARY is set by bin/brew
+# shellcheck disable=SC2154
+
+normalise_tap_name() {
+  local dir="$1"
+  local user
+  local repo
+
+  user="$(echo "${dir%%/*}" | tr '[:upper:]' '[:lower:]')"
+  repo="$(echo "${dir#*/}" | tr '[:upper:]' '[:lower:]')"
+  repo="${repo#@(home|linux)brew-}"
+  echo "${user}/${repo}"
+}
+
+homebrew-tap() {
+  case "$1" in
+    # check we actually have tap and not e.g. tapsomething
+    tap) ;;
+    tap*) return 1 ;;
+    *) ;;
+  esac
+
+  # Named args are handled by the Ruby code.
+  if [[ "$#" -gt 1 ]]
+  then
+    return 1
+  fi
+
+  local taplib="${HOMEBREW_LIBRARY}/Taps"
+  (
+    shopt -s extglob
+
+    for dir in "${taplib}"/*/*
+    do
+      [[ -d "${dir}" ]] || continue
+      dir="${dir#"${taplib}"/}"
+      normalise_tap_name "${dir}"
+    done | sort
+  )
+}