Those '^' chars are valid to reference a relative revision.
Example:

  hg log -r tip^1

  hg log -r NODEID^5
If the reviewboard requests the old revision via hgweb it will fail

since hgweb does not support the relative revision syntax. So we

need to rewrite this relative node id to an absolute node id.
Also it will modify short and valid negative revision numbers to

full changeset node id as short revision numbers are local only.
hg log -r -1

   hg log -r 123
Bugs closed: 3915