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