Refactor ConfigEntry and related iterators for safer memory management

[beamerblvd]

ConfigEntry is holding on to git_config_entry pointers for its entire lifespan, but this isn't safe or necessary. When getting a single entry without an iterator, this isn't really a concern. However, when entries are iterated with git_config_next, the previously-allocated iterator entry object is re-used. If ConfigEntry is still holding onto the git_config_entry, its values could change inappropriately and unexpectedly. The solution to this is to eagerly copy the underlying C values into the Python structure and move on.

Additionally, git_config_entry_free should never be used for iterated entries. Freeing the iterator automatically frees any entries it allocated, and manually freeing the entry can cause a crash. git_config_entry_free should be called only for entries retrieved without an iterator.

This pull request, which is a subset of the changes initially proposed in #1473, fixes this memory misuse by refactoring ConfigEntry and its related iterators.