bin/brew: put commands into 'homebrew-' namespace

This should help to avoid collisions with external commands and other
shell functions in the future and is closer to what we do in Ruby, where
commands are namespaced by being methods of the `Homebrew` module.

Library/Homebrew/cmd/update-bash.sh

@@ -220,7 +220,7 @@ pull() {
   trap - SIGINT
 }
 
-update-bash() {
+homebrew-update-bash() {
   local option
   local DIR
   local UPSTREAM_BRANCH

bin/brew

@@ -164,7 +164,7 @@ then
   # Hide shellcheck complaint:
   # shellcheck source=/dev/null
   source "$HOMEBREW_BASH_COMMAND"
-  { "$HOMEBREW_COMMAND" "$@"; exit $?; }
+  { "homebrew-$HOMEBREW_COMMAND" "$@"; exit $?; }
 else
   exec "$HOMEBREW_RUBY_PATH" -W0 "$HOMEBREW_LIBRARY/brew.rb" "$@"
 fi