Operations on promisor objects make sense in the context of only a
small subset of the commands that internally use the revisions
machinery, but the "--exclude-promisor-objects" option were taken
and led to nonsense results by commands like "log", to which it
didn't make much sense.  This has been corrected.

* md/exclude-promisor-objects-fix:
  exclude-promisor-objects: declare when option is allowed
  Documentation/git-log.txt: do not show --exclude-promisor-objects