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

As GitPython is in maintenance mode, there will be no new features.
However, I believe it's good idea to explicitly state we do not support
certain things if this is the case.

Therefore, when worktrees are encountered, we will throw an specific
exception to indicate that.

The current implementation is hacky to speed up development,
and increases the risk of failing due to false-positive worktree
directories.

Related to #344

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`