Merge branch 'master' into next

* master:
  Don't pass "--xhtml" to hightlight in gitweb.perl script.
  rebase -i: clarify in-editor documentation of "exec"
  tests: sanitize more git environment variables
  fast-import: treat filemodify with empty tree as delete
  rebase: give a better error message for bogus branch
  rebase: use explicit "--" with checkout