Merge branch 'en/rebase-root-and-fork-point-are-incompatible'

Incompatible options "--root" and "--fork-point" of "git rebase"
have been marked and documented as being incompatible.

* en/rebase-root-and-fork-point-are-incompatible:
  rebase: display an error if --root and --fork-point are both provided

diff --cc Documentation/git-rebase.txt
Simple merge

diff --cc builtin/rebase.c
Simple merge