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