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.

[jdavid]

Thanks. This is the review from Kimi:

1. No tests for the fix. A memory-safety refactor of this size (+201/−93 in config.py) ships with zero test changes. Please add regression tests, e.g.: collect list(config) and assert values remain intact after iteration ends; interleave two iterators and assert earlier entries aren't corrupted; valueless-key get_bool behavior. These are exactly the scenarios from Access violation accessing repo config entries #970.

2. Docstring: multivar "first value" is wrong. The new get_bool/get_int docstrings say "If the config var found is a multivar, only the first value is returned." Verified: git_config_get_entry returns the last value (['first', 'last'] → get_entry gives 'last'), matching git config --get semantics.

3. Docstring: "empty value" listed as truthy is wrong. get_bool/parse_bool say truthy values include "an empty value / NULL". Verified: Config.parse_bool('') is False (empty string is falsy); only a missing/NULL value is truthy. Suggest "a missing value (NULL)".