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).

Now 2 more TCs pass in Windows:
+ TestRepo.test_file_handle_leaks()
+ TestObjDbPerformance.test_random_access()

See https://github.com/gitpython-developers/smmap/pull/30

+ Decygpath sm's `.git` file contents.
+ Polish another path in `git add`; actually no main-code changes, just
a replace \-->/ on a relative(!) path to make cygwin-git to work.

- REGRESSION `test_git_submodules_and_add_sm_with_new_commit` asks for
user/email settings.

- Cygwin TCs failing:
  - PY2: err: 2, fail: 1
  - PY3: err: 2, fail: 1

+ Use environment-markers in requirement files (see
http://stackoverflow.com/a/33451105/548792).

- Cygwin TCs failing:
  - PY2: err: 13, fail: 2
  - PY3: err: 12, fail: 2

+ Parse most remote & config-urls \-->/.
+ Used relative daemon-paths.
+ Use git-daemon PORT above 10k; on Windows all below need Admin rights.
+FIXED git-daemon  @with_rw_and_rw_remote_repo():
  + test_base.test_with_rw_remote_and_rw_repo() PASS.
  + test_remote.test_base() now freezes! (so still hidden win_err)

+ repo_test: minor finally delete test-repos created inside this repo.
+ util: delete unused `absolute_project_path()`.

+ Mark another TC failing when not in master.