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.

Related to #383

Previously, it could have happened that pipes ran full, deadlocking the operation

Related to #72

This should fix resource leaking issues once and for all.

Related #304

Wrap `git merge-base --is-ancestor` into its own function because it acts
as a boolean check unlike base `git merge-base call`

I knew that flake would eventually get me, especially when least
suspected.
This time it's even useful, as it is a non-whitespace related issue.