+ Bug appeared as last 5 TCs (test_commit & test_stream) said:

  OSError: [WinError 6] The handle is invalid

+ Detect code breaking the body of TCs eventually hidden win-errors by
raising SkipTest ALAP.
+ submodule.base.py: import classes from `git.objects` instead of
`utils`.
+ had to ++ ulimit 100->110 for the extra code tested (more leaks :-)
+ Centralize is_win detection.

+ Some cases had restructuring of code.

+ 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. 

+ Cannot `index.path` into ENV, it is bytes!
+ The hook TC never runs on linux!
+ Unblock removal of odbfile in perf-large streams TC.
+ Attempt to unblock removal of submodule file by intensive cleaning.

more unblock files

Without this commit the update() function of a submodule does not work
in CentOS 6.

When the code is run without setting up loggers, the loggers have no
handlers for the emitted messages. The logging module displays:
`No handlers could be found for logger "git.cmd"` on the
console. By adding a NullHandler (a no-op) the message disappears,
and doesn't affect logging when other handlers are configured.