Reset to remote master when updating git clones

When installing with --HEAD, cached repo clones do "git fetch origin"
followed by "git reset --hard". This will only reset the working tree
and index to the state of the local HEAD, but Homebrew will think it's
updated and install anyway. "git reset --hard origin/master" will
achieve the desired result.

Should fix Homebrew/homebrew#7613.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/download_strategy.rb

@@ -346,7 +346,8 @@ class GitDownloadStrategy < AbstractDownloadStrategy
       else
         # otherwise the checkout-index won't checkout HEAD
         # https://github.com/mxcl/homebrew/issues/7124
-        quiet_safe_system "git", "reset", "--hard"
+        # must specify origin/master, otherwise it resets to the current local HEAD
+        quiet_safe_system "git", "reset", "--hard", "origin/master"
       end
       # http://stackoverflow.com/questions/160608/how-to-do-a-git-export-like-svn-export
       safe_system 'git', 'checkout-index', '-a', '-f', "--prefix=#{dst}/"