+ FIX TestRepo.test_submodule_update():
  + submod: del `.git` file prior overwrite; Windows denied otherwise!
+ FIX TestRepo.test_untracked_files():
  + In the `git add <file>` case, it failed with unicode args on PY2.
Had to
encode them with `locale.getpreferredencoding()` AND use SHELL.
+ cmd: add `shell` into `execute()` kwds, for overriding USE_SHELL per
command.
+ repo: replace blocky `communicate()` in `_clone()` with thread-pumps.
+ test_repo.py: unittestize (almost  all) assertions.
+ Replace open --> with open for index (base and TC).
+ test_index.py: Enabled a dormant assertion. 

+ No WindowsError exception.
+ Add `test_exc.py` for unicode issues.
+ Single-arg for decoding-streams in pump-func.

+ TCs: unittest-asserts for git-tests.

Relying on the destructor will not work, even though
the code used to rely on it.

Now we handle failures more explicitly. Far from perfect,
but a good start for a fix.

Fixes #514

Issue #407

It was a left-over of some prior hacking that was not removed by
accident.

* untracked_files could, if there were spaces in the path returned,
  re-rencode the previously decoded unicode string thanks to a
  `decode("string_escape")` call. Now re-encode into utf-8 afterwards
  - added test to assure this works indeed
* IndexFile.add() didn't handle unicode correctly and would write
  broken index files. The solution was to compute the path length after
  encoding it into utf-8 bytes, not before ... .

Closes #320