Turns out aec58a9 did the right thing for /packed/ refs, but didn't work
correctly on /unpacked/ refs.  So this patch gives unpacked refs the
same treatment.

Without the fix here, the test added will cause this traceback:

======================================================================
ERROR: Check that we find .git as a worktree file and find the worktree
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/pjones/devel/github.com/GitPython/git/test/lib/helper.py", line 92, in wrapper
    return func(self, path)
  File "/home/pjones/devel/github.com/GitPython/git/test/test_repo.py", line 938, in test_git_work_tree_dotgit
    self.assertIsInstance(repo.heads['aaaaaaaa'], Head)
  File "/home/pjones/devel/github.com/GitPython/git/util.py", line 893, in __getitem__
    raise IndexError("No item found with id %r" % (self._prefix + index))
IndexError: No item found with id 'aaaaaaaa'

Woops.

Things I've learned:
- test_remote doesn't work currently if you start on a branch.  I think
  it never did?
- Because of 346424da, all *sorts* of stuff in the test suite doesn't
  work if you name your development branch "packed-refs"
  (This seems like a bug...)

Signed-off-by: Peter Jones <pjones@redhat.com>

This makes Repo("foo") work when foo/.git is a file of the form created
by "git worktree add", i.e. it's a text file that says:

gitdir: /home/me/project/.git/worktrees/bar

and where /home/me/project/.git/ is the nominal gitdir, but
/home/me/project/.git/worktrees/bar has this worktree's HEAD etc and a
"gitdir" file that contains the path of foo/.git .

Signed-off-by: Peter Jones <pjones@redhat.com>

https://docs.python.org/3/whatsnew/3.6.html#deprecated-python-behavior

Details:

- Added Python 2.6 again to .travis.yml (it was removed in commit 4486bcb).

- Replaced the use of dictionary comprehensions in `git/cmd.py` around
  line 800 with the code before that change (in commit 25a2ebf).
  Reason: dict comprehensions were introduced only in Python 2.7.

- Changed the import source for `SkipTest` and `skipIf` from `unittest.case`
  to first trying `unittest` and upon ImportError from `unittest2`.
  This was done in `git/util.py` and in several testcases.
  Reason: `SkipTest` and `skipIf` were introduced to unittest only
  in Python 2.7, and `unittest2` is a backport of `unittest` additions
  to Python 2.6.

- In git/test/lib/helper.py, fixed the definition of `assertRaisesRegex`
  to work on py26.

- For Python 2.6, added the `unittest2` dependency to `requirements.txt`
  and changed `.travis.yml` to install `unittest2`. Because git/util.py
  uses SkipTest from unittest/unittest2, the dependency could not be added
  to `test-requirements.txt`.

- Fixed an assertion in `git/test/test_index.py` to also allow
  a Python 2.6 specific exception message.

- In `is_cygwin_git()` in `git/util.py`, replaced `check_output()` with
  `Popen()`. It was added in Python 2.7.

- Enabled Python 2.6 for Windows:

  - Added Python 2.6 for MINGW in .appveyor.yml.

  - When defining `PROC_CREATIONFLAGS` in `git/cmd.py`, made use of certain
    win32 and subprocess flags that were introduced in Python 2.7, dependent
    on whether we run on Python 2.7 or higher.

  - In `AutoInterrupt.__del__()` in `git/cmd.py`, allowed for `os` not having
    `kill()`. `os.kill()` was added for Windows in Python 2.7 (For Linux, it
    existed in Python 2.6 already).