+ 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. 

+ Modify lock/read-config-file code to ensure files closed.
+ Use `with GitConfigarser()` more systematically in TCs.
+ Clear any locks left hanging from prev Tcs.
+ Util: mark lock-files as SHORT_LIVED; save some SSDs...

+ TCs: unittest-asserts for git-tests.

It's entirely untested if this repo still does the right thing,
but I'd think it does.

Fixes #504

is_dirty supports path. Fixes #482.

That way, we will not only get the summary line
contained in the blame, but fetch the full message.
This is more costly than the previous implementation
allowed it to be, but being less surprising/correct
certainly is the preferred behaviour here.

Fixes #485