doctor: use minimum Git version set in brew.sh

Library/Homebrew/brew.sh

@@ -105,7 +105,8 @@ then
     HOMEBREW_FORCE_BREWED_CURL="1"
   fi
 
-  # The system Git is too old for some Homebrew functionality we rely on.
+  # The system Git on macOS versions before Sierra is too old for some Homebrew functionality we rely on.
+  HOMEBREW_MINIMUM_GIT_VERSION="2.14.3"
   if [[ "$HOMEBREW_MACOS_VERSION_NUMERIC" -lt "101200" ]]
   then
     HOMEBREW_FORCE_BREWED_GIT="1"
@@ -198,6 +199,7 @@ export HOMEBREW_SYSTEM
 export HOMEBREW_CURL
 export HOMEBREW_SYSTEM_CURL_TOO_OLD
 export HOMEBREW_GIT
+export HOMEBREW_MINIMUM_GIT_VERSION
 export HOMEBREW_PROCESSOR
 export HOMEBREW_PRODUCT
 export HOMEBREW_OS_VERSION

Library/Homebrew/diagnostic.rb

@@ -489,8 +489,7 @@ module Homebrew
       end
 
       def check_git_version
-        # System Git version on macOS Sierra.
-        minimum_version = "2.14.3".freeze
+        minimum_version = ENV["HOMEBREW_MINIMUM_GIT_VERSION"].freeze
         return unless Utils.git_available?
         return if Version.create(Utils.git_version) >= Version.create(minimum_version)
 

Library/Homebrew/system_config.rb

@@ -141,6 +141,7 @@ class SystemConfig
         HOMEBREW_LIBRARY
         HOMEBREW_MACOS_VERSION
         HOMEBREW_MACOS_VERSION_NUMERIC
+        HOMEBREW_MINIMUM_GIT_VERSION
         HOMEBREW_RUBY_PATH
         HOMEBREW_SYSTEM
         HOMEBREW_SYSTEM_TEMP