rebase: use a better reflog message

Now that the "checkout" invoked internally from "rebase" knows to
honor GIT_REFLOG_ACTION, we can start to use it to write a better
reflog message when "rebase anotherbranch", "rebase --onto branch",
etc. internally checks out the new fork point.  We will write:

  rebase: checkout master

instead of the old

  rebase

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -497,7 +497,9 @@ then
 	if test -z "$force_rebase"
 	then
 		# Lazily switch to the target branch if needed...
-		test -z "$switch_to" || git checkout "$switch_to" --
+		test -z "$switch_to" ||
+		GIT_REFLOG_ACTION="$GIT_REFLOG_ACTION: checkout $switch_to" \
+			git checkout "$switch_to" --
 		say "$(eval_gettext "Current branch \$branch_name is up to date.")"
 		exit 0
 	else
@@ -522,7 +524,9 @@ test "$type" = interactive && run_specific_rebase
 
 # Detach HEAD and reset the tree
 say "$(gettext "First, rewinding head to replay your work on top of it...")"
-git checkout -q "$onto^0" || die "could not detach HEAD"
+
+GIT_REFLOG_ACTION="$GIT_REFLOG_ACTION: checkout $onto_name" \
+	git checkout -q "$onto^0" || die "could not detach HEAD"
 git update-ref ORIG_HEAD $orig_head
 
 # If the $onto is a proper descendant of the tip of the branch, then