update: improve repo_var

* use HOMEBREW_REPOSITORY instead of HOMEBREW_PREFIX
* better performance:
  * update-bash: avoid shellout
  * update-report: use strip_prefix and tr
* more robust:
  * explicitly handle the case when repo is HOMEBREW_REPOSITORY to avoid
    to handle the trailing backslash.
  * handle both lower case and upper case when stripping non alpha and
    digital characters.

Library/Homebrew/cmd/update-bash.sh

@@ -45,11 +45,15 @@ git_init_if_necessary() {
 }
 
 repo_var() {
-  echo "$1" |
-    sed -e "s|$HOMEBREW_PREFIX||g" \
-        -e 's|Library/Taps/||g' \
-        -e 's|[^a-z0-9]|_|g' |
-    tr "[:lower:]" "[:upper:]"
+  local repo_var="$1"
+  if [[ "$repo_var" = "$HOMEBREW_REPOSITORY" ]]
+  then
+    repo_var=""
+  else
+    repo_var="${repo_var#"$HOMEBREW_LIBRARY/Taps"}"
+    repo_var="$(echo -n "$repo_var" | tr -C "A-Za-z0-9" "_" | tr "[:lower:]" "[:upper:]")"
+  fi
+  echo "$repo_var"
 }
 
 upstream_branch() {

Library/Homebrew/cmd/update-report.rb

@@ -151,11 +151,14 @@ class Reporter
   attr_reader :initial_revision, :current_revision, :repository
 
   def self.repository_variable(repository)
-    repository.to_s.
-      gsub("#{HOMEBREW_PREFIX}", "").
-      gsub("Library/Taps/", "").
-      gsub(/[^a-z0-9]/, "_").
-      upcase
+    if repository == HOMEBREW_REPOSITORY
+      ""
+    else
+      repository.to_s.
+        strip_prefix(Tap::TAP_DIRECTORY.to_s).
+        tr("^A-Za-z0-9", "_").
+        upcase
+    end
   end
 
   def initialize(repository)