git_packbuilder_write: Allow setting path to NULL to use the default path

We should not be in the business of copying strings around for users.
We either return a strarray that can be freed, or we take one (and do
not mutate it).

We _dispose_ the contents of objects; we _free_ objects (and their
contents).  Update `git_strarray_free` to be `git_strarray_dispose`.
`git_strarray_free` remains as a deprecated proxy function.

If given a NULL path, write to the object path of the repository.

Add tests for the new behavior.

Provide macros to replace usages of `assert`.  A true `assert` is
punishing as a library.  Instead we should do our best to not crash.

GIT_ASSERT_ARG(x) will now assert that the given argument complies to
some format and sets an error message and returns `-1` if it does not.

GIT_ASSERT(x) is for internal usage, and available as an internal
consistency check.  It will set an error message and return `-1` in the
event of failure.

refdb_backend: improve callback documentation

The credential structures are now opaque and defined in
`sys/credential.h`.  However, we should continue to provide them for
backward compatibility, unless `GIT_DEPRECATED_HARD` is set.

The callbacks are currently sparsely documented, making it really hard
to implement a new backend without taking a look at the existing
refdb_fs backend. Add documentation to make this task hopefully easier
to achieve.