Some Git command line options are allowed to be repeated multiple times.
Examples of this are the -C flag which may occur more than once to
"strengthen" its effect, or the -L flag on Git blames, to select
multiple blocks of lines to blame.

    $ git diff -C -C HEAD~1 HEAD
    $ git blame -L 1-3 -L 12-18 HEAD -- somefile.py

This patch supports passing a list/tuple as the value part for kwargs,
so that the generated Git command contain the repeated options.

Seems like OSX is somewhat special here ... .

Fixed additional test which seems to have different outcomes depending
on the interpreter. This just makes it work withouth attempting
to find the root cause of the issue.

This allows us to use the main thread to parse stderr to get progress,
and resolve assertion failures hopefully once and for all.

Relates to #301

Test was adjusted as well to parse only a single file which simulates
stderr output.

* Previously we could fail to parse the last line within a read buffer,
  which is now fixed.
* Added a test to verify our *slow* line parsing works as expected.

It shows that the previous implementation was never really working on
linux, and thus failed on travis as well for good reason.

Closes #303

In that case, the handler for processing stdout and stderr of the git
process is offloaded to threads. These currently don't return
any exception they raise.

We could easily fix this using an approach as shown
[here](http://goo.gl/hnVax6).

Related to #248