objectname::
The object name (aka SHA-1).
+upstream::
+ The name of a local ref which can be considered ``upstream''
+ from the displayed ref. Respects `:short` in the same way as
+ `refname` above.
+
In addition to the above, for commit and tag objects, the header
field names (`tree`, `parent`, `object`, `type`, and `tag`) can
be used to specify the value in the header field.