diff --git a/Library/Homebrew/dev-cmd/update-test.rb b/Library/Homebrew/dev-cmd/update-test.rb
index eed09234b5..d87572ff29 100644
--- a/Library/Homebrew/dev-cmd/update-test.rb
+++ b/Library/Homebrew/dev-cmd/update-test.rb
@@ -46,19 +46,24 @@ module Homebrew
         Utils.popen_read("git", "rev-list", "-n1", "--before=#{date}", "origin/master").chomp
       elsif args.to_tag?
         tags = Utils.popen_read("git", "tag", "--list", "--sort=-version:refname")
-        previous_tag = tags.lines.second
-        previous_tag ||= begin
-          if (HOMEBREW_REPOSITORY/".git/shallow").exist?
+        if tags.blank?
+          tags = if (HOMEBREW_REPOSITORY/".git/shallow").exist?
             safe_system "git", "fetch", "--tags", "--depth=1"
-            tags = Utils.popen_read("git", "tag", "--list", "--sort=-version:refname")
+            Utils.popen_read("git", "tag", "--list", "--sort=-version:refname")
           elsif OS.linux?
-            tags = Utils.popen_read("git tag --list | sort -rV")
+            Utils.popen_read("git tag --list | sort -rV")
           end
-          tags.lines.second
         end
+        current_tag, previous_tag, = tags.lines
+        current_tag = current_tag.to_s.chomp
+        odie "Could not find current tag in:\n#{tags}" if current_tag.empty?
+        # ^0 ensures this points to the commit rather than the tag object.
+        end_commit = "#{current_tag}^0"
+
         previous_tag = previous_tag.to_s.chomp
         odie "Could not find previous tag in:\n#{tags}" if previous_tag.empty?
-        previous_tag
+        # ^0 ensures this points to the commit rather than the tag object.
+        "#{previous_tag}^0"
       else
         Utils.popen_read("git", "rev-parse", "origin/master").chomp
       end
@@ -67,8 +72,13 @@ module Homebrew
       start_commit = Utils.popen_read("git", "rev-parse", start_commit).chomp
       odie "Could not find start commit!" if start_commit.empty?
 
-      end_commit = Utils.popen_read("git", "rev-parse", "HEAD").chomp
+      end_commit ||= "HEAD"
+      end_commit = Utils.popen_read("git", "rev-parse", end_commit).chomp
       odie "Could not find end commit!" if end_commit.empty?
+
+      if Utils.popen_read("git", "branch", "--list", "master").blank?
+        safe_system "git", "branch", "master", "origin/master"
+      end
     end
 
     puts "Start commit: #{start_commit}"
@@ -80,10 +90,12 @@ module Homebrew
 
       oh1 "Setup test environment..."
       # copy Homebrew installation
-      safe_system "git", "clone", "--local", "#{HOMEBREW_REPOSITORY}/.git", "."
+      safe_system "git", "clone", "#{HOMEBREW_REPOSITORY}/.git", ".",
+                  "--local", "--branch", "master", "--single-branch"
 
       # set git origin to another copy
-      safe_system "git", "clone", "--local", "--bare", "#{HOMEBREW_REPOSITORY}/.git", "remote.git"
+      safe_system "git", "clone", "#{HOMEBREW_REPOSITORY}/.git", "remote.git",
+                  "--local", "--bare", "--branch", "master", "--single-branch"
       safe_system "git", "config", "remote.origin.url", "#{curdir}/remote.git"
 
       # force push origin to end_commit
@@ -100,7 +112,7 @@ module Homebrew
       oh1 "Running brew update..."
       safe_system "brew", "update", "--verbose"
       actual_end_commit = Utils.popen_read("git", "rev-parse", branch).chomp
-      if start_commit != end_commit && start_commit == actual_end_commit
+      if actual_end_commit != end_commit
         raise <<~EOS
           brew update didn't update #{branch}!
           Start commit:        #{start_commit}
@@ -110,6 +122,6 @@ module Homebrew
       end
     end
   ensure
-    FileUtils.rm_r "update-test" unless args.keep_tmp?
+    FileUtils.rm_rf "update-test" unless args.keep_tmp?
   end
 end