Instead of copying over the data into the individual entries, point to
the originals, which are already in a format we can use.

In C89 it is undefined behavior to pass `NULL` pointers to
`strncmp` and later on in C99 it has been explicitly stated that
functions with an argument declared as `size_t nmemb` specifying
the array length shall always have valid parameters, no matter if
`nmemb` is 0 or not (see ISO 9899 §7.21.1.2).

The function `str_equal_no_trailing_slash` always passes its
parameters to `strncmp` if their lengths match. This means if one
parameter is `NULL` and the other one either `NULL` or a string
with length 0 we will pass the pointers to `strncmp` and cause
undefined behavior.

Fix this by explicitly handling the case when both lengths are 0.

Remove most of the silly warnings

This fits with the style for the rest of the project, but more
importantly, makes life easier for bindings authors who auto-generate
code.

Reload the HEAD and index data for a submodule after reading the
configuration.  The configuration may specify a `path`, so we must
update HEAD and index data with that path in mind.