option was given. In same way as to for `only` it can be followed
by an equal sign and explicit value. E.g.,
`%(trailers:only,unfold=true)` unfolds and shows all trailer lines.
+** 'valueonly[=val]': skip over the key part of the trailer line and only
+ show the value part. Also this optionally allows explicit value.
NOTE: Some placeholders may depend on other options given to the
revision traversal engine. For example, the `%g*` reflog options will