strarray: we should `dispose` instead of `free`
We _dispose_ the contents of objects; we _free_ objects (and their
contents). Update `git_strarray_free` to be `git_strarray_dispose`.
`git_strarray_free` remains as a deprecated proxy function.
refs::namespace: add namespace tests
These simple tests only ensure that we enforce the existence of a
namespace; these mirror the rugged tests, they are not exhaustive.