Merge branch 'jk/grep-haystack-is-read-only'

Code clean-up in the "grep" machinery.

* jk/grep-haystack-is-read-only:
  grep: store grep_source buffer as const
  grep: mark "haystack" buffers as const
  grep: stop modifying buffer in grep_source_1()
  grep: stop modifying buffer in show_line()
  grep: stop modifying buffer in strip_timestamp