fix(cmd): fixed deadlock when stderr buffer overflow

Fixed deadlock when using stderr=PIPE in Popen and Git generates enough
output to a pipe such that it blocks waiting for the OS pipe buffer to
accept more data (see
https://docs.python.org/2/library/subprocess.html#subprocess.Popen.wait)

Previously it was possible for it to pick up non-repository
branch configuration, even though it was unlikely.

Closes #350

Similar to git, we now ignore options which have no value.
Previously it would not handle it consistently, and throw a parsing
error the first time the cache was built.

Afterwards, it was fully usable though.
Now we specifically check for the case of no-value options instead.

Closes #349

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

fix(cmd): make short options with arguments become two separate argum…