Opt to split lines by the new line character instead of letting
`splitlines()` do this. This helps catch the issue when there are
special characters in the line, particular the commit summary section.

Fixes #430

Fixes #428

Returning this now to avoid having to change the function's return value
structure later on if we want to emit more information.

This can be used to pass options like -C or -M.

This adds a sibling method to Repo's blame method:

    Repo.blame_incremental(rev, path, **kwargs)

This can alternatively be called using:

    Repo.blame(rev, path, incremental=True)

The main difference is that blame incremental is a bit more efficient
and does not return the full file's contents, just the commits and the
line number ranges.  The parser is a bit more straight-forward and
faster since the incremental output format is defined a little stricter.

if used as context managers, the parsers will automatically release their file locks.