This issue only surfaced in python 2, in case paths containing unicode
characters were not actual unicode objects.

In python 3, this was never the issue.

Closes #331

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

When expanding directories, check if it is a symlink and don't expand
them at all.

Previously, we followed symlinks and expanded their contents, which
could lead to weird index files.

Fixes #302

This should prevent a resource warning given in py3

Fixes #263

It turned out that the index is not actually corrupted, which is good
news. What happens is that `git` writes `TREE` extension data into the
index, which causes it to write out the given tree *as is* next time
a `git commit` is executed. When using `git add`, this extension data
is maintained automatically. However, GitPython doesn't do that ... .
Usually this is no problem at all, as you are supposed to use
`IndexFile.commit(...)` along with `IndexFile.add(...)`.

Thanks to a shortcoming in the GitPython API, the index was
automatically written out whenever files have been added, without
providing control over whether or not *extension data* will be
written along with it.

My fix consists of an additional flag in `IndexFile.add(...)`, which
causes extension data not to be written by default, so commits can be
safely done via `git commit` or `IndexFile.commit(...)`.

However, this might introduce new subtle bugs in case someone is
relying on extension data to be written. As this can be controlled
through the said flag though, a fix is easily done in that case.

Fixes #265

Related to #224
[ci skip]