Improve handling of ProcessContextGitMock.outputMessages

[hohwille]

ProcessContextGitMock has an internal state from which it builds the ProcessResult.
Tests can add outputMessages to it that are accumulated.
The problem is that sub-sequent invocations with this approach lead to strange effects because the second invocation will additionally get output messages from the first invocation.

Therefore we should do the following:

• When we create the ProcessResult we create a new List from outputMessages as a copy
• At the end of the run method before we return the ProcessResult we do this.outputMessages.clear() to rest the state and avoid such ugly side-effects.