FormulaInstaller: reset git_available? when installing git

Closes Homebrew/homebrew#43693

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/formula_installer.rb

@@ -451,6 +451,9 @@ class FormulaInstaller
 
     ohai "Summary" if verbose? || show_summary_heading?
     puts summary
+
+    # let's reset Utils.git_available? if we just installed git
+    Utils.clear_git_available_cache if formula.name == "git"
   ensure
     unlock
   end

Library/Homebrew/utils/git.rb

@@ -21,5 +21,11 @@ module Utils
     rescue
       raise "Git is unavailable"
     end
+
+    raise "Git is unavailable" unless git_available?
+  end
+
+  def self.clear_git_available_cache
+    remove_instance_variable(:@git) if instance_variable_defined?(:@git)
   end
 end