This namespace is about behaving like git's branch command, so let's do
exactly that instead of taking a reflog message.
This override is still available via the reference namespace.
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.
This adds giterr_user_cancel to return GIT_EUSER and clear any
error message that is sitting around. As a result of using that
in places, we need to be more thorough with capturing errors that
happen inside a callback when used internally. To help with that,
this also adds giterr_capture and giterr_restore so that when we
internally use a foreach-type function that clears errors and
converts them to GIT_EUSER, it is easier to restore not just the
return value, but the actual error message text.
