Communication over pipe made more robust.

[mitar]

Previously, communication with git could get into an inconsistent state if there was some exception while communicating. And exceptions are: sometimes split fails (because the request is for missing commit, for example), or some other exception is thrown.

Now, the communication is broken so that it can be restarted from a clean slate next time. As this does not happen so often, I don't think it makes a negative impact on performance. But once it does happen, is better that things work than to get an exception displayed to the user that something internally went wrong.

[hvr]

Please consider getting in touch with the upstream Trac development
(i.e. submit this directly on t.e.o), as the git plugin has been merged
upstream for 0.13.x and development for the github-hosted 0.12 is
stalled due to lack of time...