....
'commit' SP <ref> LF
mark?
- ('author' SP <name> SP LT <email> GT SP <when> LF)?
- 'committer' SP <name> SP LT <email> GT SP <when> LF
+ ('author' (SP <name>)? SP LT <email> GT SP <when> LF)?
+ 'committer' (SP <name>)? SP LT <email> GT SP <when> LF
data
('from' SP <committish> LF)?
('merge' SP <committish> LF)?
....
'tag' SP <name> LF
'from' SP <committish> LF
- 'tagger' SP <name> SP LT <email> GT SP <when> LF
+ 'tagger' (SP <name>)? SP LT <email> GT SP <when> LF
data
....