rebase: delete obsolete internal "keepopen" option
The option was apparently introduced for use by the "pbranch"
extension, see
f2558a8228be (rebase: add option to not commit after a
collapsing, 2010-02-07). However, it doesn't seem like it was ever
used by that extension (according to `hg grep` in a clone of
https://bitbucket.org/parren/hg-pbranch/), so let's delete it.
Differential Revision: https://phab.mercurial-scm.org/D2708
#require docutils gettext
$ $TESTDIR/check-gendoc el
checking for parse errors