bin/brew: don't pass command name to bash commands

We have asserted before that the 1st argument is the command name. No
need to pass it to the bash command, which will make the argument
handling for the command itself a bit easier.

bin/brew

@@ -153,6 +153,9 @@ fi
 
 if [[ -f "$HOMEBREW_BASH_COMMAND" ]]
 then
+  # Bash commands don't need the first argument, which is just the command name.
+  shift
+
   # source rather than executing directly to ensure the entire file is read into
   # memory before it is run. This makes running a Bash script behave more like
   # a Ruby script and avoids hard-to-debug issues if the Bash script is updated