rebase --root: stop assuming squash_onto is unset

If the user set the environment variable 'squash_onto', the 'rebase'
command would erroneously assume that the user passed the option
'--root'.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -60,6 +60,7 @@ $(gettext 'Resolve all conflicts manually, mark them as resolved with
 You can instead skip this commit: run "git rebase --skip".
 To abort and get back to the state before "git rebase", run "git rebase --abort".')
 "
+squash_onto=
 unset onto
 unset restrict_revision
 cmd=