update-report: cleanup git harder in migration.

Before stashing also try to cancel any current merge, rebase and
reset. This will prevent any data loss but also make the stash more
likely to succeed.

Library/Homebrew/cmd/update-report.rb

@@ -208,6 +208,9 @@ module Homebrew
 
     unless Utils.popen_read("git status --untracked-files=all --porcelain").empty?
       HOMEBREW_REPOSITORY.cd do
+        quiet_system "git", "merge", "--abort"
+        quiet_system "git", "rebase", "--abort"
+        quiet_system "git", "reset", "--mixed"
         safe_system "git", "-c", "user.email=brew-update@localhost",
                            "-c", "user.name=brew update",
                            "stash", "save", "--include-untracked"