grep: build list of columns without "grep." label prefix Prepares for formatter support. We need field names without "grep.".

grep: factor out function that prints matched line with labels Prepares for formatter support.

grep: refactor loop that yields matched text with label As preparation for formatter support, this and the next patch split linestate.__iter__() into two functions, line scanner and displayer. New code uses regexp.search(str, pos) in place of regexp.search(substr), which appears to fix a bug of highlighting.

Added signature for changeset ccd436f7db6d