When duplicating a configuration entry, we allocate a new entry but do
not verify that we get a valid pointer back. As we're dereferencing the
pointer afterwards, we might thus run into a segfault in out-of-memory
situations.

Extract a new function `git_config_entries_dup_entry` that handles the
complete entry duplication. Fix the error by using
`GIT_ERROR_CHECK_ALLOC`.