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.
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.
