`HOMEBREW_CURL_PATH` has an effect only when `HOMEBREW_DEVELOPER` is set. However, the part of `brew.sh` that prints a message about outdated cURL disregards the value of `HOMEBREW_DEVELOPER`, which leads to a misleadnig message telling the user that `HOMEBREW_CURL_PATH` is outdated even though another cURL was used/tested.
This PR fixes it and instructs Homebrew to:
1. Display a warning message when system cURL is outdated and either `HOMEBREW_CURL_PATH` **or `HOMEBREW_DEVELOPER`** are not set. New `HOMEBREW_CURL_WARNING` variable is set to display the above warning only once (useful when `brew` calls itself internally).
2. Display `Installing Homebrew cURL` before auto-installing cURL in `update.sh` (due to `HOMEBREW_FORCE_BREWED_CURL`) and stop/exit if this step fails.
3. Display `Installing Homebrew Git` before auto-installing Git in `update.sh` (due to `HOMEBREW_FORCE_BREWED_GIT`) and stop/exit if this step fails.
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