When running a Coverity build, we have to provide an
authentication token in order to proof that we are actually
allowed to run analysis in the name of a certain project. As this
token should be secret, it is only set on the main repository, so
when we were requested to run the Coverity script on another
repository we do error out. But in fact we do also error out if
the Coverity analysis should _not_ be run if there is no
authentication token provided.

Fix the issue by only checking for the authentication token after
determining if analysis is indeed requested.

We used to only execute Coverity analysis on the 'development'
branch before commit 998f001 (Refine build limitation,
2014-01-15), which refined Coverity build limitations. While we
do not really use the 'development' branch anymore, it does
still make sense to only analyze a single branch, as otherwise
Coverity might get confused.

Re-establish the restriction such that we only analyze libgit2's
'master' branch. Also fix the message announcing why we do not
actually analyze a certain build.

The Mac machines have updated their SSH version and so the ssh-keygen
format has changed. Ask it for MD5, which is the one that is output as
hex.

This lets us see the details of what we're doing instead of just seeing the
output of unknown commands in the build output.

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.