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.