The code accessing config entries in the `git_config_entries` structure
is still much too intimate with implementation details, directly
accessing the maps and handling indices. Provide two new functions to
get config entries from the internal map structure to decouple the
interfaces and use them in the config file code.

The function `git_config_entries_get` will simply look up the entry by
name and, in the case of a multi-value, return the last occurrence of
that entry. The second function, `git_config_entries_get_unique`, will
only return an entry if it is unique and not included via another
configuration file. This one is required to properly implement write
operations for single entries, as we refuse to write to or delete a
single entry if it is not clear which one was meant.