As GitPython is in maintenance mode, there will be no new features.
However, I believe it's good idea to explicitly state we do not support
certain things if this is the case.

Therefore, when worktrees are encountered, we will throw an specific
exception to indicate that.

The current implementation is hacky to speed up development,
and increases the risk of failing due to false-positive worktree
directories.

Related to #344

This should fix resource leaking issues once and for all.

Related #304

Wrap `git merge-base --is-ancestor` into its own function because it acts
as a boolean check unlike base `git merge-base call`

I knew that flake would eventually get me, especially when least
suspected.
This time it's even useful, as it is a non-whitespace related issue.

I have no idea why PY3 requires such a mess of encoding/decoding
statements, but let's just be happy it works.

Also let's be sure I never ever write python code again ... EVER.

* 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

revA..revB → revA...revB (three instead of two dots) [base.py, line
467](https://github.com/gitpython-developers/GitPython/blob/master/git/repo/base.py#L467)

rorepo is a ~~a~~ Repo instance [test_docs.py, line
21](https://github.com/gitpython-developers/GitPython/blob/master/git/test/test_docs.py#L21)
closes #314

Adds the optional keyword parameter env to Repo.clone_from(). The parameter
is a dictionary containing the desired environment variables for the git
clone invocation. The environment is applied to the temporary Git instance
before calling Repo._clone().