When no reference names could be found we did error out when trying to describe
a commit. This is wrong, though, when the option to fall back to a commit's
object ID is set.

The signature for the reflog is not something which changes
dynamically. Almost all uses will be NULL, since we want for the
repository's default identity to be used, making it noise.

In order to allow for changing the identity, we instead provide
git_repository_set_ident() and git_repository_ident() which allow a user
to override the choice of signature.

If there is a failure then cl_git_pass tries to get the libgit2
error, but p_... functions don't set that.

Also - trailing whitespace cleanup.

And implement the option init functions for this and the format options.

This makes us be closer to git's tests, and lets us better describe what
we expect from the output.

When we describe the workdir, we perform a describe on HEAD and then
check to see if the worktree is dirty. If it is and we have a suffix
string, we append that to the buffer.

Instead of printing out to the buffer inside the information-gathering
phase, write the data to a intermediate result structure.

This allows us to split the options into gathering options and
formatting options, simplifying the gathering code.

We don't describe arbitrary object, so let's give it the name of the one
object type we accept.