coverity: add user model

coverity: add user model

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.