Merge branch 'rs/maint-grep-F' into maint

Merge branch 'rs/maint-grep-F' into maint

"git grep -e '$pattern'", unlike the case where the patterns are read from
a file, did not treat individual lines in the given pattern argument as
separate regular expressions as it should.

By René Scharfe
* rs/maint-grep-F:
  grep: stop leaking line strings with -f
  grep: support newline separated pattern list
  grep: factor out do_append_grep_pat()
  grep: factor out create_grep_pat()