Merge branch 'rs/grep-p'

Merge branch 'rs/grep-p'

* rs/grep-p:
  grep: simplify -p output
  grep -p: support user defined regular expressions
  grep: add option -p/--show-function
  grep: handle pre context lines on demand
  grep: print context hunk marks between files
  grep: move context hunk mark handling into show_line()
  userdiff: add xdiff_clear_find_func()