update: fix hyphen counting (legacy tap renaming)

Fix regression introduced in fafe8f0f53bf91fc41f016b5c2af41ca712783f7.
Counting the number of hyphens in a string cannot be done in a single
expression, thus split this and introduce another local variable.

Fixes #227.

Library/Homebrew/cmd/update.sh

@@ -40,6 +40,7 @@ git_init_if_necessary() {
 rename_taps_dir_if_necessary() {
   local tap_dir
   local tap_dir_basename
+  local tap_dir_hyphens
   local user
   local repo
 
@@ -55,7 +56,8 @@ rename_taps_dir_if_necessary() {
       mkdir -p "$HOMEBREW_LIBRARY/Taps/$user"
       mv "$tap_dir" "$HOMEBREW_LIBRARY/Taps/$user/homebrew-$repo"
 
-      if [[ ${#${tap_dir_basename//[^\-]}} -gt 1 ]]
+      tap_dir_hyphens="${tap_dir_basename//[^\-]}"
+      if [[ ${#tap_dir_hyphens} -gt 1 ]]
       then
         echo "Homebrew changed the structure of Taps like <someuser>/<sometap>." >&2
         echo "So you may need to rename $HOMEBREW_LIBRARY/Taps/$user/homebrew-$repo manually." >&2