A particular commit can have an UUID or an Instant when it occurred, or
both. Also this information is something we would like to include in
Serializable contexts -- and therefore we promote CommitInfo to a
non-Beta Serializable.
JIRA: YANGTOOLS-1559
Change-Id: I1a2eb7f41484c4088e6ce310b3ab78a26022ad40 Signed-off-by: Robert Varga <robert.varga@pantheon.tech>