always prefer HOMEBREW_CURL over plain 'curl'

Library/Homebrew/cmd/update.sh

@@ -304,7 +304,7 @@ EOS
         UPSTREAM_BRANCH_LOCAL_SHA="$(git rev-parse "refs/remotes/origin/$UPSTREAM_BRANCH")"
         # Only try to `git fetch` when the upstream branch is at a different SHA
         # (so the API does not return 304: unmodified).
-        UPSTREAM_SHA_HTTP_CODE="$(curl --silent '--max-time' 3 \
+        UPSTREAM_SHA_HTTP_CODE="$("$HOMEBREW_CURL" --silent '--max-time' 3 \
            --output /dev/null --write-out "%{http_code}" \
            --user-agent "$HOMEBREW_USER_AGENT_CURL" \
            --header "Accept: application/vnd.github.chitauri-preview+sha" \

Library/Homebrew/utils/analytics.rb

@@ -16,8 +16,8 @@ def report_analytics(type, metadata={})
   # any personally identifiable information.
   # https://developers.google.com/analytics/devguides/collection/protocol/v1/devguide
   # https://developers.google.com/analytics/devguides/collection/protocol/v1/parameters
-  system "curl", "https://www.google-analytics.com/collect", "-d", "v=1",
-    "--silent", "--max-time", "3", "--output", "/dev/null",
+  system ENV["HOMEBREW_CURL"], "https://www.google-analytics.com/collect",
+    "-d", "v=1", "--silent", "--max-time", "3", "--output", "/dev/null",
     "--user-agent", "#{HOMEBREW_USER_AGENT_CURL}",
     "-d", "tid=#{ENV["HOMEBREW_ANALYTICS_ID"]}",
     "-d", "cid=#{ENV["HOMEBREW_ANALYTICS_USER_UUID"]}",

Library/brew.sh

@@ -218,7 +218,7 @@ then
   # information.
   # https://developers.google.com/analytics/devguides/collection/protocol/v1/devguide#screenView
   # https://developers.google.com/analytics/devguides/collection/protocol/v1/parameters
-  curl https://www.google-analytics.com/collect -d v=1 \
+  "$HOMEBREW_CURL" https://www.google-analytics.com/collect -d v=1 \
     --silent --max-time 3 --output /dev/null \
     --user-agent "$HOMEBREW_USER_AGENT_CURL" \
     -d tid="$HOMEBREW_ANALYTICS_ID" \