We need to keep hold of the strings which we create. We must also hold
on to the array of strings which we assing to our git_strarray.
We were not doing the latter, which meant that our strings may have been
freed too early, leaving us with with memory access errors (though often
not leading to a crash due to the custom allocator in python).
As we need to keep hold of two/three pieces of information, this looks
like a good place to introduce a context manager. This allows us to
keep these pointers alive without burdening the call sites with a return
of multiple objects they have no use for.
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