update: restore previous state when interrupted

Fixes Homebrew/homebrew#19460.
Closes Homebrew/homebrew#22375.

Library/Homebrew/cmd/update.rb

@@ -86,11 +86,19 @@ class Updater
     # the refspec ensures that 'origin/master' gets updated
     args << "refs/heads/master:refs/remotes/origin/master"
 
-    safe_system "git", *args
+    reset_on_interrupt { safe_system "git", *args }
 
     @current_revision = read_current_revision
   end
 
+  def reset_on_interrupt
+    ignore_interrupts { yield }
+  ensure
+    if $?.signaled? && $?.termsig == 2 # SIGINT
+      safe_system "git", "reset", "--hard", @initial_revision
+    end
+  end
+
   # Matches raw git diff format (see `man git-diff-tree`)
   DIFFTREE_RX = /^:[0-7]{6} [0-7]{6} [0-9a-fA-F]{40} [0-9a-fA-F]{40} ([ACDMR])\d{0,3}\t(.+?)(?:\t(.+))?$/