grep: display correct user/revision for --all in reverse.
Also handle multiple occurrences of the same line by storing
linestates in an array instead of a dict, and using difflib
to compute the difference between linestates.
abort: Failed to parse .../t/.hg/hgrc
File contains no section headers.
file: .../t/.hg/hgrc, line: 1
'invalid\n'