Write debug files for stderr output as well as FETCH_INFO into
the git repository, e.g. into `.git/`

Help with #301

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 will make usage of the `NullHandler` possible in python 2.6
and below.

When the code is run without setting up loggers, the loggers have no
handlers for the emitted messages. The logging module displays:
`No handlers could be found for logger "git.cmd"` on the
console. By adding a NullHandler (a no-op) the message disappears,
and doesn't affect logging when other handlers are configured.

support passing odbt for using with Repo

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).

Reverted changes of `fe2fbc5~2`.

This caused `git-pull` to error, which now actually results in a fatal
error while fetching or pulling. Previously we simply didn't check
for this issue.

Now we are back to a `poll` based or threaded concurrent reading from
stdout and stderr to prevent a git process deadlock, and the
aforementioned error.

Related to #297