Closes #330

There is simply no excuse to doing that.

Closes #330

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

The previous patch failed to update the expected number of op_codes,
which would result in an exception when creating an instance of
RemoteProgress. This patch corrects the value to the new expected number
of op_codes (8)

When running a long running operation (such as a clone on a large repo),
Git may return a message indicating that it is 'Finding sources'. Since
there is no bit field value for this message, this causes a large amount
of error messages to be emitted to stderr.

This patch fixes this by adding another bit field value for this
message, FINDING_SOURCES. Derived classes can look for this op_code and
handle it appropriately.

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

When pushing/pulling, we ignore errors unless it's exit code 128.
The reason for this is now made explicit to make clear that issues
are handled by PushInfo flags accordingly.

Related #271

Previously, it checked for AssertionErrors, now we have to implement
need-unbare-repo check ourselves.