Coverity user model

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.

It takes a bit for the propxy to get ready to accept connections, so
start it before the build so we can be reasonably sure that it's going
to be ready in time.

Coverity fixes round 6

The tests have never run successfully and we do have successful builds
of mingw-w64, so remove these CI builds which do not add value.

Curl by default does not report errors by setting the error code.
As the upload can fail through several conditions (e.g. the rate
limit, leading to unauthorized access) we should indicate this
information in Travis CI.

To improve upon the behavior, use `--write-out=%{http_code}` to
write out the HTTP code in addition to the received body and
return an error if the code does not equal 201.