git-rebase.sh: update help messages a bit

signoff is not specific to the am-backend.  Also, re-order a few options
to make like things (e.g. strategy and strategy-option) be near each
other.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -20,23 +20,23 @@ onto=!             rebase onto given branch instead of upstream
 r,rebase-merges?   try to rebase merges instead of skipping them
 p,preserve-merges! try to recreate merges instead of ignoring them
 s,strategy=!       use the given merge strategy
+X,strategy-option=! pass the argument through to the merge strategy
 no-ff!             cherry-pick all commits, even if unchanged
+f,force-rebase!    cherry-pick all commits, even if unchanged
 m,merge!           use merging strategies to rebase
 i,interactive!     let the user edit the list of commits to rebase
 x,exec=!           add exec lines after each commit of the editable list
 k,keep-empty	   preserve empty commits during rebase
 allow-empty-message allow rebasing commits with empty messages
-f,force-rebase!    force rebase even if branch is up to date
-X,strategy-option=! pass the argument through to the merge strategy
 stat!              display a diffstat of what changed upstream
 n,no-stat!         do not show diffstat of what changed upstream
 verify             allow pre-rebase hook to run
 rerere-autoupdate  allow rerere to update index with resolved conflicts
 root!              rebase all reachable commits up to the root(s)
 autosquash         move commits that begin with squash!/fixup! under -i
+signoff            add a Signed-off-by: line to each commit
 committer-date-is-author-date! passed to 'git am'
 ignore-date!       passed to 'git am'
-signoff            passed to 'git am'
 whitespace=!       passed to 'git apply'
 ignore-whitespace! passed to 'git apply'
 C=!                passed to 'git apply'