Coverity considers that anything that looks like assert() behaves like
it (ie. side-effects would be skipped on a NDEBUG build). As we have a
bunch of those in the test suite (128), this would ensure Coverity isn't
confused.

The `git_buf` structure seems to be too complicated to correctly
grasp for Coverity. As such, add simpler models trying to guide
Coverity and remove false positives related to these functions.

The static analysis engine coverity allows for user models
overriding how it treats functions when analyzing code. Like
this, one can greatly reduce the rate of false positives and thus
make it easier to spot actual errors.

Add a user model that overrides function models for `git_buf_len`
and `git_vector_insert`, which together amount for a majority of
false positives.