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

It turns out we can't deal do fetches if no refspec is set as git
will change the format of the fetch return values, providing less
information than usual.

A test was added to show that such a case will fail, and an assertion
will assure we don't attempt to fetch/pull if there is no refspec
for 'fetch'.

Closes #296

In that case, the handler for processing stdout and stderr of the git
process is offloaded to threads. These currently don't return
any exception they raise.

We could easily fix this using an approach as shown
[here](http://goo.gl/hnVax6).

* config parser now handles quoted values correctly. This doesn't hamper
  multi-line support.
* added regression test to travis to assure we will be warned if we
  rewrite and break the user's .gitconfig file
* only rewrite configuration files if we actually called a mutating
  method on the writer. Previously it would always rewrite it.

Fixes #285

add a GIT_PYTHON_TEST_GIT_DAEMON_PORT to set a port other than 9418,
for example for when you already have a daemon running on that port.

No reason to expose a daemon to all interfaces when it is only used for
tests, which connect to localhost anyway.
I'd love to use localhost here instead, but the git-daemon man page points out:

    If IPv6 is not supported, then --listen=hostname is also not
    supported and --listen must be given an IPv4 address.

I don't know of a way to check if git has ipv6 support, but 127.0.0.1
should be around for the foreseeable future

It expected to see major version 0 though.

Related to #248

Do not swallow non-zero exit status during push and fetch unless
we managed to parse head information.

This behaviour will effetively handle cases were no work was done
due to invalid refspecs or insufficient permissions.

Fixes #271